I have never been able to find info on CG approval/endorsement/preference
for any particular flotation.

snip
the USCG regulation two part and
snip

Can you cite some kind of reference for this?

The only relevant thing I have seen is at
http://www.uscg.mil/d8/mso/louisvill...bp16761_3b.pdf
page 39. Wish I could cut and paste the text here, but the gist is that
there are many appropriate materials and techniques, some better than
others. The only restrictions seem to be that fuel-resistant foam must be
used near the engine and in bilges, and the hull can not be used as part of
an air chamber. (So I fill the chambers with soda bottles).

Sal's Dad