Conditioning after ferment in multiple glass carboys is what I had in mind, although I don't see why you couldn't use another Sanke keg. Two six gallon or 6.5 gallon carboys would do the trick for a 13 gallon brew minus trub losses.
I agree on the multiple carboys for the secondaries. I'll be using Better Bottles.
I disagree on using a Sanke keg for the secondary for 12 gallons of beer. There would be too much headspace and it would oxidize.
I have changed my find on Sankes as primary fermentors. I just measured my plastic pails and they are about the same overall diameter as the Sanke kegs. The plastic pails have a 1" lip on them and thus they take more room than the Sankes. The kegs are taller. I could fit one keg with 12-13 gallons of beer in just about the same space that a plastic pail would take.
I'm going to cut one and try it as a primary fermentor. A medium to large sized deep freeze would make an excellent lagering chamber. One could fit 2 Sanke kegs (24 gallons of beer) plus carboys and serving kegs in it. And they are cheap if you buy them used.
Note that if you can pressurize the keg, then you can void the oxygen out of it easily with C02 so headspace would be irrelevant. Actually, nitrogen would probably work best for that purpose.
I guess you could do that. I'm not 100% certain that gets rid of all the O2. Add a touch of sugar to the brew or racking before its done fermentation and it will self purge.
I am going to leave off the racking valve, so that they fit in the deep freeze better. One could make a lid with a port for a racking cane and force a little air/CO2 in to start the flow. I do that with my Better Bottles now. It works great.