I think most people here intuitively disagree with this, and assume your "evidence" (in the sense of the word that comes into Bayesian reasoning) includes only what you "know" in the subjective, introspective sense, not the externalist sense.

Yeah, I'm getting this now, and I must admit I'm surprised. I had assumed that accepting some form of semantic externalism is obviously crucial to a fully satisfactory naturalistic epistemology. I still think this is true, but perhaps it is less obvious than I thought. I might make a separate post defending this particular claim.

You're right that the BB-based skeptical argument you offer is a different argument for skepticism than brains-in-vats. I'm not sure it's a more serious argument, though. The second premise in your argument ("If current physics is not essentially correct, I know nothing about the universe.") seems obviously false. Also the implication that I am very likely to be a BB does not come just from current physics. It comes from current physics in conjunction with something like SSA. So there's a third horn here, which says SSA is incorrect. And accepting this doesn't seem to have particularly dire consequences for our epistemological status.