I don't see this question clearly asked and answered anywhere, it's certainly not in the ServerFault FAQ where I was trying to find it.
From reading some of the stuff on meta, it seems that:
the correct way to deal with a question on ServerFault that should be on SuperUser is to flag it for moderator attention with a note that the question belongs elsewhere.
the wrong thing to do is to slap on a "belongs-on-superuser" tag.
Is that about right?