Piecemeal armor variant rules were never implemented from Ultimate Equipment.
I'm unsure as to whether to do this as part of the equipment mod system or another way.
TEMPBONUS System, as this seems geared towards a character trying to get dressed in a hurry, and wouldn't be commonly used.
Mind, these would be penalties applied based upon the choice.
I'd be interested in some variant of this simply for Equipment slot exclusions. When equipping a set of Full Plate, PCGen generates a dummy Gauntlet to equip in my Unarmed slot (very nifty, btw,) but there's no dummy Visored Helm, Padded (under)Armor, or Sturdy Leather Boots which are also included with a set of Full Plate. (Other sets/suits of heavy and some medium armor have similar issues)