This is what I would do in that situation:
Make sure the CO2 tank is off and off the connector from your gas line so that you just have an open gas line coming from the regulator. Then open the keg lid, slide it to the side while keeping it in the keg, and hold the line above the beer and slowly open the tank to let some CO2 enter the keg and pushing the air out. Then spray a bunch of starsan all over the dam thing, close it up, and leave it for Tuesday when you can get the fitting. When you get the fitting, hook it up and purge it very well.
Basically, you are going the same thing most people do when bottling. CO2 is more sense than air, so it should push the air out and leave a nice CO2 blanket over the beer, which should be good for a few days as long as you don't open it.
EDIT: looks like we all were basically typing the same thing around the same time. Great minds think alike, but only superb minds think of the correct answer.