Well, async let is built into the language, so if we want to start the initializer on its formal isolation, we'll have to handle that in the built-in logic for async let. I agree that we should! But we don't actually need new language mechanics like @isolated(any) to make that happen — we could always have handled that in the built-in logic for async let. I think the real question here is what the isolation of the async let initializer is, because (I think) we currently always treat it as non-isolated. We probably need a better answer than that in general because of sendability.
I don't know if it makes sense to try to answer that in this proposal, though.