I think Shalvenay's answer from the Draconomicon is the only official answer you're going to get, and it may well be good enough for your purposes. But, I feel like giving an alternative answer anyway, based on the SRD and applied to all levels of manoeuvrability.
Turning Rules
Here is the relevant SRD text (with one more entry than the question included), followed by my expanded explanations. Comments on Perfect manoeuvrability are excluded, because Perfect fliers have no turning restrictions whatsoever.
Turn
How much the creature can turn after covering the stated distance.
To turn while moving forwards, a creature needs to move forwards a certain distance (10 ft. for Clumsy fliers, 5 ft. otherwise). They can then add a turn to their movement for no additional movement cost (90° for Good fliers, 45° otherwise)
Turn in Place
A creature with good or average maneuverability can use some of its speed to turn in place.
At a cost of 5 ft. of movement speed, Good fliers can turn 90° without needing to move out of their square at all; Average fliers can only do 45° for the same cost.
Maximum Turn
How much the creature can turn in any one space.
Related to the above. Good fliers are able to hover, and so their ability to turn in place is only limited by their movement speed. Average fliers are limited to a 90° turn in place (two 45° turns, at a cost of 10 ft. of movement), and will have to move forwards at least a little bit before they can turn again. Also, since they can't hover, they have to have maintained their minimum forward speed once all their turning is done, or else fall out of the sky.
Presumably, this turning limitation applies whether the first 45° was a turn after movement or a turn in place. This would be why Poor and Clumsy fliers are limited to 45° in one space: that's how much they're allowed to turn after movement, and they can't add a turn-in-place to that.
So, conclusions from the above rules...
Perfect
Perfect manoeuvrability has no RAW limitations on turning. You can do a 1080° spin in your square for zero movement cost, if you want to.
Hence, the Perfect turning radius is 0 ft.
Good
At a cost of 5 ft. of movement for every 90°, a creature with Good manoeuvrability can complete an otherwise unlimited turn in place, giving them a turning radius of 0 ft.
If moving forwards instead of hovering (i.e. not using turn in place at all), they can turn sharply enough to do a complete 360° in four squares, making their turn radius 5 ft. in that case.
Average
The tightest turn a creature with Average manoeuvrability can manage is a series of 45° turns in place, 5 ft. forwards movements, and 45° turns. (This also guarantees that they'll maintain their minimum forward speed, since it's 5 ft. of genuine movement for every 5 ft. of "lost" movement.)
Hence, they can manage a 90° turn for every 5 ft. of forwards movement, just like Good fliers, but at a total cost of 10 ft. of movement speed; their turn radius is 5 ft.
Without turn in place, an Average flier is reduced to exactly the same turning options as a Poor flier; see below.
Poor
A creature with Poor manoeuvrability cannot turn in place at all. For every 5 ft. of forwards movement, they can turn 45°, allowing them to do a complete 360° in a space 20 ft. wide: a turning radius of 10 ft.
Clumsy
Creatures with Clumsy manoeuvrability turn like those with Poor manoeuvrability, except that they need 10 ft. of forwards movement instead of 5 ft. They can manage to turn a complete 360° in a space 35 ft. wide, for a turning radius of 17.5 ft.
I was concerned that the intricacies of diagonal movement might have affected that answer (because one diagonal movement might have been 10 ft. in one square, allowing them to stop moving diagonally sooner), but it turns out that it doesn't--as long as we assume that they start with their "diagonal movement counter" at zero, so their first diagonal only costs 5 ft.
Diagram
Here's a picture:
Key:
- Black arrows are forwards movement combined with a free turn.
- Red arrows are turns in place.
- Numbers give the movement cost of the adjacent arrow.
- Asterisks mark diagonal movements.
- Circled letters give the manoeuvrability shown by each loop: Clumsy, Poor, Average, or Good (with and without turns in place).
- For Perfect manoeuvrability, just picture a whirlwind of red arrows.
Anaruoch: The Empire of Shade includes a 50-ft.-tall, 10-ft.-wide waterfall inside a cavern. Features of the Area describes the waterfall as follows:
Climbing beneath the waterfall requires a successful DC 35 Climb check. Flying through it requires a successful DC 20 Strength check to avoid plummeting to the pool below. (73)
Extrapolating from this, a creature that wants not to be swept away when confronted with a suddenly flooding passageway should probably allowed to make a Strength check (DC 20) if the creature has means of going against the deluge in the first place (e.g. appropriate handholds, a fly speed, an active effect like the spell spider climb, extreme weight due to the spell iron body). That is, effectively the creature has chosen to try to stay in place while the waterfall flows over the creature. Success means the creature can take its actions normally; I suggest failure means the creature's swept away (rather than having the creature plummet).
Success!
After a successful Strength check made to avoid being swept away, forward progress against this ersatz waterfall should require a climbing creature to make Climb skill checks (DC 35). (Note: While neither hydrologist nor spider-eating spelunker, I think that DC's about right considering the potential water speeds were discussing. I don't know if matters if failure means you technically fall or, instead, are technically swept away as both involve facing tons of water coming right at you! I'm guessing, but at this point traversing the horizontal seems like it should be as difficult as traversing the vertical. Comments can take up this issue, and, of course, the DM can adjust the DC as he sees fit.)
How other creatures make forward progress is up to the DM, but flying creatures likely must swim (perhaps in stormy waters), and super heavy creatures likely use their land speed (albeit maybe vastly reduced—this is, after all, pretty difficult terrain). All such creatures probably also need to hold their breath.
Failure!
After a failed Strength check, the creature's swept away. Aquatic Terrain under Flowing Water describes being swept away as follows:
Characters swept away by a river moving 60 feet per round or faster must make DC 20 Swim checks every round to avoid going under. If a character gets a check result of 5 or more over the minimum necessary, he arrests his motion by catching a rock, tree limb, or bottom snag—he is no longer being carried along by the flow of the water. Escaping the rapids by reaching the bank requires three DC 20 Swim checks in a row. Characters arrested by a rock, limb, or snag can’t escape under their own power unless they strike out into the water and attempt to swim their way clear. Other characters can rescue them as if they were trapped in quicksand (described in Marsh Terrain, above).
Emphasis mine, and which should cover equally well a rapidly flooding cavern.
Best Answer
You can choose to just fall, if you want.
There’s nothing saying that you have to use your singular action to stay aloft, you just need to if you, ya know, want to say aloft.
Some sources of flight specify the ability to glide or use wings as parachutes of sorts, even when paralyzed or what have you (see Dragon Wings from Races of the Dragon). This is not the default however. In the absence of such a feature...
Yes, the dragon falls straight down, taking the appropriate falling damage, unless he spends his singular standard-or-move action maintaining forward momentum.