There are some filaments designed for investment casting that are considered low ash... and IIRC, it is PLA based. Avoid ABS, obviously, but it seems with good ventilation, PLA shouldn't be an issue. Then again, can you add a step by making a wax cast from a PLA master... or any printed master?

Maker's Muse did a flameout test with various materials. Nothing scientific but fun to watch. He was focused on self-extinguishing.
I'll see if I can find the investment casting video again.

Oh, and I've seen chocolate printers!