What's the equivalent of NetworkingPeer.OpCustom() when inside a room? (using LoadBalancing + PUN)

I was able to add code to the Server's MasterClientPeer to handle custom operations when a client is either in a lobby or simply on the MasterServer. Whenever I enter a room though, I get an error when trying to send an OpCustom() saying I can't send them (yet) because of my state (I'm neither on MasterServer nor Lobby). I realize that I'm technically on a GameServer whenever I'm in a room, but I thought it would work. Is there an equivalent function of OpCustom() for when I'm inside a room? I thought maybe RaiseEvent() was what I needed, but I don't quite get where to customize the Server code. Any help is appreciated! Thanks

edit: I essentially need a way to execute some code on the server when the client's room is full.