The problem with this is that if we think a game might be filling up then time is a critical factor. We’re not going to want to wait while we send you a PM and wait for a response.
How about something like, NEW NOTE: Please don’t hesitate to submit an entry even if the thread has more than 50 responses. “Game Over” will be added to the thread title when the game is full.
You may get a handful of late entries now and then, but I haven’t seen anyone get bent out of shape over that.