I'm not convinced on the benefits of this yet, but I think it is worthy of discussion.

Overlays are technically complicated, and not really mobile-friendly, adding this could cause more clunkiness and confusion than it would help in speed and intuitiveness.
It's also a risk with themeing as site themers would need to ensure they don't break the standard overlay mechanism in their theme.

I imagine a user finding an edit form via the CMS zone, and finding an edit overlay, and getting kind of lost due to the 2 different contexts for the same thing. In my mind having 1 way of doing any thing is overall simpler.

Ah didn't realize it would be complicated for themeing