Now I get ya Ebb.
How about airbags strategically placed in these "emergency" voids, which could be deployed when needed, then repacked when all is well... Kind of like an air bag in a car.
I understand that styrofoam has better floatation than the equivalent volume of air - makes me wonder if it wouldn't be better to permanently attach enough of the stuff in the nooks and crannies to make the boat positively buoyant - if that's possible.
Although, the styrofoam option would take up a constant amount of volume, whereas the airbags in their deflated state would take up much less valuable volume...