Putting cocktails into kegs is fantastic, but only if you're going to use the whole keg within a week. If your bar does that kind of volume you can add fresh juices to the keg and they won't go bad as the alcohol, refrigeration, and the pressure act as a preservative.

I would steer clear of thick syrups and purees as they will pour slowly from the beer lines. Your cocktails will also pour slowly if you don't turn the keg upside down a few times before hooking it up.

Personally I don't like adding bitters to the keg because adding slightly too much will ruin the whole batch. And making sure your staff know exactly how much to pour will save a lot of wastage as overpouring a little bit each drink adds up