I'm under the impression that OSHA requires the grounded conductor to be opened. Portable GFCI have this capability. I believe I read it from ECM or something similar. I'll look for it
OSHA requires both open neutral and overvoltage protection in portable GFCIs. This is why spider boxes have the separate GFCI modules and not GFI breakers or receptacles. The open neutral protection prevents the GFCI from having voltage on its load side if the supply neutral is lost (which would prevent a normal GFCI from functioning) or is at a high voltage.