I have "watered down" a brew in a 3 gallon keg by mixing in 40oz commercial beers.
It worked. The BMC drinkers sucked that thing dry, quick. It tasted OK to me.
I just carbed the crap out of the beer in the keg overnight, put the bottles right next to the mouth of the keg, and gently poured. I hit the keg with a shot of CO2 a couple of times during the pouring. I was aiming to form a "pillow" of CO2 to help keep the O2 away.