OK, if I understand this right you want something like the old "May West" type life jacket rigged to self inflate if you need to ditch?
Kind of like the EPIRBs (Emergency Position Indicating Radio Beacons) on ships are supposed to do.
It might even be able to rig it so it attaches to the outside of the kit.
I think the idea of compact flotation for a survival kit is a worthwhile idea to play with.

Even though when I boat or canoe my packs are all positively buoyant and sealed water tight the sleeping bags and clothes are bulky, so something less bulky for flying would be very desirable.

You gave a weight of 25 pounds. Since water weighs about 62 pounds to a cubic foot you should be OK for the 25 pounds with 1/2 foot of volume.
If you had 1 foot of volume it should float both you and your pack.

I am considering this idea of yours now. A PFD with a small CO2 cylinder should do it quite nicely.

Edit:
Jeff seems to have it roped,tied and branded.