If you are starting with a new or good insert that doesn't yet need rebuilding, I keep mine soaked with mineral oil. This replaces smearing Vaseline on broadheads to keep them from rusting, as Vaseline is a mild blood coagulant. It also keeps the foam pliable, makes it last much longer and doesn't stink.
Seems like your spray insulation idea is a workable rebuild, assuming it will bond to the old foam ... or are you talking about removing the old foam and then replacing with spray? Give it a try and let us know. dave