Zane,
I was thinking of doing the same. I used to paddle whitewater kayaks and those flotation bags were tough. I'm also considering using playground kick balls to fill the back two chambers on the BBIII. Water will inevitably find its way into these compartments, especially if you have your bow/stern lights mounted into them. Removing two-part foam on my BBIII took me 15+ hours to complete, so I doubt I'll ever want to go that route again.
Grace, peace, and good times with your project this summer!
David