Development Assistance Setting Max Players

OK, so I've run into a problem where I'm sort of lost in a cycle of looking at unhelpful documentation and finding bad examples online.
What I want to do is very simply this: Set the max number of players that can join the server, without changing the config and reloading. I know it can be done, because somehow or another MinePlex does it. (No, I'm not copying MinePlex -- first off, I'm writing my own code; secondly, I'm adding other stuff to this dynamic server idea.)
So, since I'm finding getMaxPlayers under Server, but no set, I'm wondering: Is what I'm asking possible with online mode? Or would it require some nasty packet manipulation?