My idea (using the good stuff) would be to pack the empty space with the cured fingers of firestop, then use firesleeve clamped over the outside of the pass-through & wire bundle for CO block.
The idea is that as long as there's no fire, there's no mess; if there is a fire, the fingers will swell up & fill whatever airspace was left in the pass-through.
Sounds good to me.
Is there a downside to using a piece of firesleeve with some trimmed out lengthwise then wrapped around the wires and slid inside the tube? Similar to how the commercially available stainless pass-throughs recommend. You can then firebarrier the exposed end. Firesleeve that is manufactured to the FAA TSO can withstand 2200 deg F for 15min.
As you say, that's the principle in commercial building pass-throughs (pack with with inert fiber or similar, then seal the ends). Should work fine.
I'll toss out another approach too. Pull the wire bundle out an inch or two, wrap it tightly in aluminum tape, then push it back in so the aluminum tape is inside the sleeve. Now inject Firebarrier into the remaining space. The tape will keep it from gluing all the wires together, which is the only real PITA if you ever need to rework the harness.