'Custom' power supplies, even ones that are a simple ATX mod, are a bit tricky in legal terms since modifying a power supply can void the safety certifications.
I think the way I did the modification would be as legal as it could get as I have not broken the safety barrier of the 12V DC adapter used for the PicoPSU. Only thing I can see about it that I have connected the earth between the adapter and the 230V plug, which is not allowed unless authorized.
Safety certifications relates to the high voltage side, so if you mod an ATX PSU by just cutting the wires and not opening it, I can't see how that should be a problem.