Does drop-in-drop-out work at ALL with a GM mode, though?
I don't think that it does - especially not for the GM position. So the GM would have to be the host, and if the GM gets disconnected, that's a disconnect for everyone. I can't see different random players dropping into the middle of a GM session being great either, they'll start out confused.
In a Pen and Paper game, wouldn't it be pretty much unheard of twenty sessions in for Bob to leave, and then Clive joins the group and then starts playing Bob's character?