Can I lager in a spare Corny keg in the back of my kegerator? Can I then serve directly from that keg?
Well yes and no. Allow me to elaborate.
First, you certainly can lager in a Corny keg. That is my primary method of lagering. It works great.
There is one thing, however. I cut the beverage dip tube in the Corny keg to prevent it from sitting directly in the collected yeast that falls out during lagering. If you don't cut the dip tube, you will get a TON of yeast on transfer to a serving keg or during the first half dozen pours. Not tasty or pretty. Lagers are usually so nice and clear and pretty that it would be a shame to have all that yeast in them.
The only issue with doing it in your kegerator is the temperature movement required for lagering i.e. the constant drop by 1 degree a day until achieving lagering temperature would then effect all of the beer in your kegerator. Final lagering temperature is usually much colder than serving temperature, but I will admit that serving temperature is a personal matter.
As to serving directly out of the lagering keg, (even with a cut dip tube) I wouldn't do it for the reasons I listed above.
I have made up a quick transfer line that allows me to move the finished beer out of the lager keg into a serving keg, and that leaves behind the yeast cake.
I push it with CO2 and just hook up a picnic tap to the gas in side of the serving keg. That means the entire operation is sealed and the gasses in the serving keg have somewhere to go.
If you want to cut a dip tube, remember to file it smooth for easy re-insertion. I cut mine on a forty-five degree angle, about 2.5 to 3 inches from the bottom. Cutting wheel on a Dremel tool does the job nicely.
I don't lager in my kegerator, I lager in a small chest freezer that I have a temperature controller on. I got the little freezer for free and added the temp controller, that works well for me. Since I don't serve out of it, I didn't need to collar it. YMMV.