You can run a gas line through the drain hole in the back. I have a CO2 tank inside my 4912 with a beer gas tank running from the outside for when I use a stout tap. You will need to pull the rubber insert out of the drain hole to get the gas line to fit through it. The drain tray on the back pulls straight out, so you can remove it to put the line in and then put it back on when you're done.
I use a QD on the gas line outside the fridge so I can easily disconnect the tank if I need to move it.