I was thinking of the old aspiration of filling up the lungs of divers with the oxygen carrying fluorocarbon fluid (like in Abyss). It was deemed apparently impractical because it's too hard to breath. Well... you don't need to breathe with this.
I imagine a butt plug with two hoses, one for oxygenated fluorocarbon input, the other for output, and a cycle where fluid coming out from the butt will needed to be cleaned and re-oxygenated. The input hose could be long enough to reach the cecum (the end of the colon) and perhaps have additional holes to distribute the oxygenated fluid to the whole colon.