There will be specific tail-call support; NYF.
As for playing games with the stack frames, no, you can’t do that manually: if you can mangle the stack frame, so can a Bad Guy attacker. However, we have made threading so cheap that you can use system threads instead.
Regular continuations are essentially the same as exceptions or longjmp as far as the hardware is concerned, and are supported; NYF. Resumeable continuations we are still thinking about.