So? Unless it's on the order of 1/3^^^3, it doesn't matter how unlikely it is, and while my metaconfidence may be low for the exact value (insomuch as that means anything), it's clearly more likely than that.

Actually, you're absolutely right. I don't think it's possible to resist Pascal's mugging by discounting probabilities at the edge. I thought, initially, that you could use busy-beaver to put an upper bound on the size of claim they could express, and simply discount, at the extreme end, according to 1/BB(message length). Busy beaver would be larger than any normal mathematical function you could express in the message. Then it occurred to me that the mugger has a trivial solution:

"If you don't give me five dollars, I'm going to create (the busy beaver function of the length of this message's bitstring factorial) people, and torture them to death."

Plus, busy beaver is uncomputable, so that's not exactly trivially implementable.

EDIT: I should point out that doing what I initially proposed would be mathematical nonsense with no justification. I was just checking to see if it was possible in the trivial case.