A couple of obvious observations. The first is that I think the overall switch mechanism would become remarkably clarified by seeing how Duff's device can (or can't, but I think it would) actually operate.

The second is that if loop control gets to use labels on blocks, do other exceptions? If the answer is, "No", then we are introducing divisions again where built-ins can do things that user code cannot (easily) do. If it is, "Yes", then, well, one part of me is going, "Neato!" while another is gibbering in the corner. And the semi-rational part is left wondering what the syntax would be to break out of a nested switch to a named decision level...