I do this all of the time. I purge the CO2 a couple of times a day in the first few days after kegging and while it is pushing the CO2 back in I shake the keg. It has worked everytime for me and it gets carbonated a little faster.
I have experimented with a lot of different concentrates and have been thinking of using cranberry as well. If you go ahead with this let us know how much you used and how it turned out!
I have used these in ciders in the past. They did carbonate some, seemed to take longer to carbonate and did not carbonate to the level that I would have liked. I have found that adding 3/4 teaspoon of dextrose or brown sugar to each bottle at bottling time works best.