The one time I used resin on anything foamed is was a foam mannequin head. I thought that the paint it had would be enough, but no. It shriveled up like a raisin. I took a little while for it to do it too. Man that was something.
I don't know how it would act on foamies but you might get away with it if the PVA completely sealed it. -However- the heat may be an issue too. That I don't know for sure. The best way is to try on a sample.