I would think it's possible. I seem to recall a bridge builder in the fields that offered his son as a side kick as one of the rewards, where it read as though the son sould fight with you whenever you entered that area or something. Didn't care about think I chose a book, but if I'm remembering correctly then yes it should be possible. The (if not more) big question would be, can the game correctly handle taking him/her with you in/out of instances (any change of area, whether it be from outside to inside, out to out, in to in etc). I remember late game when Triss was following me as a companion and I exited a sewer into a house after being down there with her for quite awhile, she ran in through the front door as I came up the stairs, which was pretty stupid since that area was cut off (hence the reason for the sewer) and she was behind me when I exited the sewer to those stairs etc.
Thare is no problem with the AI - the Axii sign makes some enemies friendly to you. But the they are thick as pigshit however 