I have used Corny kegs as a secondary before. I rigged up a special blowoff tube connected to a gas ball-lock QD and draped the end of the tube into a pail of Starsan. Stupid thing sucked back over a quart of StarSan into the beer.
If I were to do it again (and I no doubt will), I would instead do as you've described and just seal it up like any other Corny keg. I'd pull the PRV every few days, but since I allow my beers to primary for 3 weeks, I don't expect much more off-gassing to occur, especially at the lower lagering temperatures. The yeast will hibernate at the near freezing temperature, and there'll be virtually no sugars left for them to work on anyway. And as you note, the keg is built to handle quite a bit of pressure anyway.