Just warm the keg to room temp (or warmer for Belgian yeast, it can handle it) and pitch an active starter in there. If you have one of the older keg lids with the plastic, dome-shaped relief valve, remove it and use an airlock with a small rubber stopper to monitor your progress. If not, just leave the relief valve open. As long as the yeast are actively fermenting when you add them, they'll go right to work and drop your gravity. The spent yeast will be drawn off the bottom of the keg with the first couple of pints.
edit:added pic for clarity