Now this I was not expecting! I thought X-Flash was deader than a dodo that's been run over by a steamroller... Guess I was wrong! I've got some time and a spare PSP, I'll test this out on 3.90 M33-3.
EDIT: No luck... No matter what I try, my PSP keeps getting "the boot". Didn't notice any option to skip the FW check. Am I just being stupid?