Tinker's Construct allows you to pull certain fluids from entities when they expire inside the Smeltery. Could you perhaps add a way for us to tweak this? Allow us to, say, extract Lava from a Spider?

not possible

Why? Couldn't you use strings for the entities?

There is no handler for it

It's hardcoded https://github.com/SlimeKnights/TinkersConstruct/blob/ac202c3a17be7491b583ad9d21ee39b7b54a51ad/src/main/java/tconstruct/smeltery/logic/SmelteryLogic.java#L324