The derating would only occur if there is a section 24 in. long or more, where the cables are "bundled" together (from my understanding).

"without maintaining spacing for a continuous length longer than 600 mm (24 in.).

If the cables are run through a joist cavity, they'll be "bundled" for 1 1/2" - 3" at the top plate, and 1 1/2" - 3" at the bottom of the wall. Each of which is far under the 24 in. limit. Where the cables dangle through the cavity, they are not wrapped, taped, or bound together. They are only "bundled" for a short distance while they enter/leave the cavity.

It may be Code, but that doesn't mean it's a wise idea!

First off, I was under a mistaken impression about service-entrance conductors, so I'll clear that up now. It turns out that the Article 100 definition of "service conductors" includes both "service-entrance conductors" from the service drop/lateral splice to the service disconnect and "service drop conductors" that go from the utility transformer or service tap ("service point" in the Code) to the drop/lateral splice that connects them to the service-entrance conductors.

Service Conductors. The conductors from the service point to the service disconnecting means.

Second, an auxiliary gutter is not a raceway (unlike the physically similar product known as "wireway") -- this is based on the informational note to the Art. 100 definition of "raceway":

Informational Note: A raceway is identified within specific article definitions.

and the fact that 366.2's definitions refer to auxiliary gutters as enclosures, not "raceways".

This means that the 230.7 prohibition does not apply to auxiliary gutters, as the service-entrance conductors and the branch circuit conductors are not in the same "cable or raceway". However, the intent of 230.7 is to keep overcurrent-protected conductors away from unprotected conductors, and my proposed configuration would not provide that segregation.

Finally, in practice, auxiliary gutters are used with panelboards as they are similar in function to switchboards or distribution centers.

Options for similar situations to mine

If you don't want to do a dumb thing that your inspector may or may not flag as it's not against Code, you have two options in a similar situation to mine:

1. Segregate the protected and unprotected conductors into separate conduits and terminate them in separate boxes

This is what Harper did to recover from his encounter with some very nasty copper thieves. This works provided you can get length in the existing cables to route them to multiple distribution boxes, and enough box space to accommodate all the splices (in addition to 4" square boxes for branches and feeders, Harper had to use a pair of 10" square NEMA boxes for incoming wiring).

2. Use divider plates in the auxiliary gutter to separate the protected and unprotected conductors.

This was Ed Beal's solution to having a feeder, branch circuits, and data wires in the same gutter -- partitions can be used to provide separated spaces within the gutter for each type of wiring. This is similar to how boxes that contain both mains and Class II or data wiring are partitioned to separate the high and low voltage sides from each other.

This has the advantage that all the wires can be routed to a single, existing point instead of having to be spread apart to route to many junction boxes, but is costlier as it may require a larger gutter to accommodate the fills within the various partitions.