Re: hand casting pewter
I've used Woods metal in latex rubber molds before (Bizmuth/tin/lead/cadmium alloy melts around 175ºF,) same gassing problem leaving a rough surface. I solved the problem by amazingly enough coming up with the idead to apply Kester's soldering paste to the inside surface of the rubber mold. The idea was the paste is used for soldering and helps the solder flow, cleans it etc.
I don't know if it will work for your higher temperature but it worked for my finely detailed medallions project so I'm thinking at the very least the metal might need something added to it when molten- which is essentially what the soldering paste did since the metal was molten in the mold for a bit and no doubt the paste which melted into it did something.