Writing this as much to have it confirmed/denied. AFAIK the 'firmware' is just the base filesystem and kernel, I know with some minimal tweaking it is possible to USB flash a backupmenu archive into a N900.

OT:
One thing that would be cool for backupmenu to become far more useful is if it could also save and flash the correct kernel along with archive so we don't end up with bootlooped N900s when we forget to make a new archive after the new kernel-power comes out.