FIMO is a polymer, it won’t hold up at glass fusing temps.
Look into making your own silicone molds for freeze and fuse. I would worry about other methods in a mickey-kiln. If a clay mold blows up in your brick kiln, it’s not that big of a deal (relatively speaking). I think it would be a bigger issue if a clay mold blew up in a microwave. You could try plaster… I think that might hold up OK.