Eralia submitted a new mod: Non-flammable materials - I mean they're all non-flammable now Read more about this mod...
An alternative, if you want flammable blocks in general, but don't want flammable structures to spawn into volcanic worlds: Saner Volcanic Worlds. The trade-off here, of course, is that it does somewhat reduce the variety of things that can spawn into volcanic worlds. I replaced some dungeon sets with ones made of non-flammable blocks, but some of the dungeon designs were just so thoroughly flammable that it didn't make aesthetic sense to replace them.