Sounds like a good idea. Might try to extend this to a general warning system if there are other areas that might benefit.
Might be able to extend the Events system to do this. For example, if some additional variables were added, like "totalSeats", "totalAvailableSeats", "totalEmptySeats", "numberOfAddedPlayers" (to differentiate between the number of players who have bought-in), and we added an Action that could display a dialog (shown to the program operator instead of the players) ... what do you think?