Hi,
Have you had a look at
Kill2090? This will allow you to reboot the Amiga and force it into either PAL or NTSC.
Or
Hackpics for how to make a hardware modification to do this.
Or
HZ for a very small assembler program to do this (with source) and
TINYpalntsc that does the same thing. Perhaps one of these could be included in a bootblock?
- Ali