When fiddling with GDM setup in Ubuntu, I finally noticed the "face browser". More specifically, I noticed how much the logic sucks. It decides whether someone is a human user by looking at the UID.
Would it not make more sense to have a "human" user group, and use that instead? I can see why "users" might not work, because some non-human accounts might need to be part of this group for access purposes, but creating a new group for humans would be a clean solution, IMO.