What happens to the bet if HPMOR ends without it being revealed who Mr.Hat&Cloak happens to be?

I think Yudkowsky is enough of a non-asshole that if that's the case, he'll consent to say whether Baba Yaga had anything to do with Hat & Cloak. Remember, he endorses betting on beliefs.

No, I am up for betting. If unforeseen plot developments make the resolution of the bet unclear, I want determining the winner to be casual and non-adversarial. I will not argue technicalities even if they could cause me to win.

What happens to the bet if HPMOR ends without it being revealed who Mr.Hat & Cloak happens to be?

That really depends. If the identity of Mr. Hat & Cloak remains unclear but Harry finds a black hat in Snape's trunk and a black cloak in Dumbledore's office, I would probably lose. If Harry figures out that Nicholas Flamel is really Baba Yaga and she's been hiding in Hogwarts all year, I would probably win. I'd be happy to let a mutual chosen third party arbitrate.

What happens to the bet if HPMOR ends without it being revealed who Mr.Hat&Cloak happens to be?

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We might agree to call the bet off by February 28th 201[5] if the bet is not resolved by then. If there's somehow a definitional dispute, we work it out, possibly with someone else on the forum arbitrating. Default option is to cancel the bet.

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So you aren't really up for betting?

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If I lose the bet I will laugh it off. You probably should not accept unless you would do the same.

I believe he(?) means he will laugh off the loss of the money, in the sense of treating it as not a big deal.