This seems pretty legit. I really can't see a downside here, other than having some sediment in the keg. Also, this method would be sweet from a prepper standpoint, when CO2 in a tank could be hard to come by... But how long would you be able to push beer, if there was no added CO2? At that point, is it basically a cask ale?
Either way, I'm going to pick up a valve and give it a shot.