So, if you have a claim like the pony claim (or Pascal's mugging), in which you have a very low estimated probability, and a very low metaconfidence, should become dramatically less likely to actually happen, in the real world, than a case in which we have a low estimated probability, but a very high confidence in that probability.

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. The human genome only takes about eight megabytes. If you want 3^^^3 of them, you'll have to get more general than "human", although only if you're certain that killing the same one 3^^^3 times doesn't count. Even if you do so, there's no way a generic pattern for producing sapient entities takes that much information.

I suspect that, in practice, it eliminates many of the Pascal's-mugging-style problems we encounter currently.

When have you last encountered one?

Personally, I think the bigger problem is a slightly related paradox in that if you take decision A, it could kill 3^^^3 people, but if you take decision B, it could kill 3^^^^3 people, but then again A could kill 3^^^^^3 people etc., so you could never make a decision. This would come up on every decision you make.