Would it be possible to have an app that was higher authority than ring0?

I know ring0 is the Kernel of the OS, so it would have to be outside the OS even. Maybe you could have something that loaded before the Windows kernel? Or even incorporated into the BIOS chip.

This would be one level better than a kernel-mode debugger.

Ring0 is the most privileged CPU mode, so you can't really achieve higher control over hardware/operating environment. Look at SoftIce, it can control all of the OS if loaded before it, but theoretically some other ring0 code can disable/damage it. Hardware debugger is the way to go if you don't want to interfere with the debugged software

what does a hardware debugger look like?

Anyone ever use one?

Check out this integrated BIOS debugger: http://www.gensw.com/pages/prod/bios/debug.htm

as you need to know, (Intl) processors has 4 mode for execution:
Real, Protected, Virtual..
IV mode is your pre-last chance

RTM, because that is what is & not more than is.

but sure, processor makers have other tip of processors,
on which they build-debug new architects.

Nice!
Another solution could be the debugging version of Bochs (which by the way is open source!).
Regards.
Bilbo