Yes in D&D 3.5e
In D&D 3.5e, a 5-foot step is not a miscellaneous action — it is not action type at all. What it is, is an option you may exercise as a consequence of not moving. The trigger to be permitted a 5-foot step is:
If you move no actual distance in a round
If that is true, you can make a 5-foot step (barring other restrictions that prevent exercising that option once permitted, of course).
Maybe in Pathfinder.
In Pathfinder, the 5-foot step is explicitly a Miscellaneous Action. Since staggered only permits free, swift, and immediate, miscellaneous actions are seemingly barred.
But when you take a step back an analyse the nature of Miscellaneous Actions, it turns out that it may not actually be an action type at all:
The following actions take a variable amount of time to accomplish or otherwise work differently than other actions.
You see this further in the other two categories (feat and skill use), which say that they take whatever actions types they say they do, or maybe none at all.
What this indicates is that "Miscellaneous Actions" isn't an action type so much as a bucket for other stuff that doesn't properly fit into the action types, the action economy, or both.
So: maybe. To interpret it as a "yes", you have to look at the Miscellaneous Actions section and conclude that a 5-foot step is not a special action type, and then you get the same conclusion as for D&D 3.5e, above.
To interpret it as a "no", you have to interpret the title of the section "Miscellaneous Actions" as creating a new action type; but this has its own issues with complicating the understanding of skill and feat use:
- is a skill/feat that says it takes a standard action simultaneously a miscellaneous as well as a standard action?
- is a skill/feat that says it takes no action simultaneously a miscellaneous action as well? is it an action or not, then?
Since the "no" results in a multiplication of mechanical entities within the rules, and they potentially lead to self-contradiction, both Occam's Razor and a hint of reductio ad absurdum leads me to favour saying "yes," a 5-foot step can be made when staggered, just like in D&D 3.5e.
Since you asked about both dnd-3.5e and pathfinder, here's the Pathfinder answer, from their FAQ
Limited actions on my turn: If an AOO or other interrupting effect reduces what actions I can take on my turn, does this reduction apply immediately?
Yes, even if it interrupts or limits your in-progress.
For example, if you are making a full attack and attempt to trip your
opponent, but you provoke an AOO because you don't have Improved Trip,
and your opponent has a spell storing weapon that's storing a hold
person, and you fail your save against the spell, you are immediately
paralyzed and can't take any of your remaining actions (including the
remainder of your full attack).
Likewise, if your opponent had the Staggering Critical feat instead of
a spell storing weapon and the attack staggered you, you would
immediately gain the staggered condition, which would prevent you from
taking any actions that violate the staggered condition's limitations.
If you provoked by taking a move action to move through the opponent's
threatened area, you could finish that move action but could not also
take a standard action after it. If you provoked as part of a full
attack (as with the trip example), becoming staggered would end your
full attack at that point and prevent you from taking a move action
after the staggering attack. It doesn't matter if the AOO happened
because of your first attack in your full attack or your last allowed
one, being staggered ends your full attack at that point because you
can't make a full attack if you're staggered.
The same logic would apply to becoming disabled or staggered from damage (rather than having the condition arbitrarily applied): You can finish the action (moving, making the one attack), but then have to immediately end your turn (since you have no actions left), even if you're in the middle of a full-attack.
Best Answer
There isn't anything in the RAW that specifically handles this, but there are (as I can see) two ways a DM can rule this.
Option 1, the simplest:
Simply change the wording of
staggered
to readThis would leave the effect of the status relatively unchanged, possibly allowing a few more actions to be performed than before.
Option 2, the specific:
With this option you change the wording to read
With the sub-note that the spell action economy can he faster, such as a quickened spell etc.
In this scenario you are pretty close to allowing everything the old/original staggered condition allowed for, but with the error margin being on the other side. In that there are some actions that were permitted in the "old" rules, that doesn't fit the new description.
Which of these two options should be used it totally up to your group, if you are indeed using the new action economy. My personal preference, however, might be with the first. Not only for its simplicity, but also because it's always better to have more options (as a player). :)
PS: There are, of course, variations and degrees of these two options. Taking rule #0 into consideration. ;)
PPS: Link to the unchained action economy