Instead of outright banning ghosts from playing ghost roles, enable event creator to alert ghosts about event that's coming up! Event creator could provide event description, list of roles needed, their expected acting and such, then make persistent notification pop up for ghosts. Similar to "Something interesting" status-effect like button, but always there as long as event is to gather roles. Then ghost players may click on that, examine roles and roll for some or all of them. Then once things are ready and there are enough ghosts, button press alerts ghosts that rolled in role and they spawn in.
"Wanna play chill pAI? Too bad, admin thinks you'd want to play as some sentient monster and screw you if you don't." - idea behind that PR is kinda horrible.