Here's a half-decent solution to the prohibition problem:
When the appletv start up, keep the SHIFT key pressed on the keyboard. It will now boot properly into Safe Mode!
This works for me *every time*, even though it won't boot without pressing shift (the same prohibition problem).
In safe mode, you can still alter the internal drive, as well as mess around with osx itself. I'm writing this on the appletv booted this way