I am thinking of this also. I made a Pilsen sMasH and it had an FG of 1.018, but I kegged it anyway. I thought the carbonation would offset the sweetness, but it has not. Now I am thinking of pulling it out of the kegerator and pitching some champagne yeast into it to dry it up, or pitch it back on an S05 yeast cake, when what's in that fermenter is done.
I had some cream soda carbed up and it took weeks for it to uncarb, but that just pulling the relief valve ring every so many days. Probably better to put a gas connector on and run it into a bottle of water, instead.