You can brush on 5 or 6 coats of joint compound (the dry powder stuff used for house 'dry wall' fabrication) and that would give you a thick enough skin to get a few vacuumpulls off your styrofoam sculpture. The thin plaster shell will protect your foam, but eventually the heat will deteriorate the foam underneath the shell and you will be in danger of crushing the plaster shell into that void area. Usually I brush on a layer every 3-6 hours. takes a couple days to cover a mold (but then again i live in the humid south, and that might be the main factor in my drying time)
If you need/want to get more vacuumpulls than a few, I would suggest you pull a mold of the sculpture, pour a Hydrostone or Ultracal plug-mold from that, and go from there.