Can't a mod add new bots to a match at a whim?
Like I suggested in my post, just have the mutator add an extra bot into the match for any player that is staying in spectator mode, and then remove said bot if/when the player joins the match. It doesn't keep the spectating player out of the roster, but it has the same result for the gameplay.
I mean there's a console command that can do that, and granted it is a little tricky to use that in LAN game but I understand it is still possible. I have a hard time imagining a mutator couldn't do that.