graycg,
Wanted to get back to you before this topic dropped off the page.
I think the idea may have merit. It would seem to me that there would be less chance of movement/migration of the filler or powder charge, when compared to the other fillers mentioned.
Some years ago there was some discussion of using styrofoam wads as "soft" gas checks. It wasn't about using them as fillers per se, but about protecting the base of PB bullets. I tried it, and found no problems. It didn't seem to make any difference either way, so I discarded the idea.
To avoid the potential problems with melted styrofoam, if there really are any, you might want to try some of the packing peanuts derived from rice products.
They aren't styrofoam, but have about the same properties, density, compressibility, etc. The way to differentiate them from styrofoam is they are round in cross section, usually slightly squiggly, somewhat off-white to a very pale tan, and when you try to "wash" one with water, it dissolves! Don't have any handy, so I can't say what the burn properties are. But, the natural fiber origin should make them more compatible with this specific use.
BTW, thanks for the insight. This gives me another arena for experimentation!