Not that I plan any extreme rafting but I'd use something like that. You could under-fill the back more than front to get you pointing the right way up
Platypus make some long zip top hydro bladders, i had thoughts of using them either side sewn into the front of a zip vest.
Tempted by the anfibio after seeing it, a similar concept but with a neck collar (of sorts) and two inflation points at mouth reach.
(and someone has made it..)
legally, or safety wise, probably not squeakily perfect.. then its worn as a back up to bouncing our bodies at speed between rocks in a tube.. by choice
Have you had any long swims?