I don't know how many times they've tried it: I've seen it done successfully with 2 or 3 different groups. As you pointed out, there are a LOT of variables: what's necessary for it to have any chance of working is, for lack of a better term, "subliminal overexposure": a few ideas presented over and over and over again on surfaces that we ignore consciously yet make note of subconsciously (I really liked the use of T-shirts on strangers.)
You're welcome. One possible way that JC could be using a form of PP reading is by implementing it in reverse. Rather than reading off the client, he gets the client to read off of him.
Another way is along the lines of the "Bodytalk" method that was developed in Australia (don't know when or who by), whereby the practitioner, having instructed the client to not answer out loud, asks verbal questions about their health while simultaneously palpating the body in order to sleuth out the condition.
All of this, if not objectively examined, could EASILY be attributed to some kind of mystical force. In a way, it kind of is: it takes lots of mental focus and lots of practice to get good at it - and the framework it's learned through will dramatically change a general practitioner's understanding of it.
JC does other stuff besides his PP abilities, such as the belly bounce (good core,) and the Magic tricks (reinforcing suggestion of mystical skill, making it easier to do his PP abilities in general; I think he was upset that the fire trick was filmed because it was prop oriented, not physiologically oriented, such as the Static Ouija Knife trick.) Presented together under the same label (ie the effects all stem from the same source, such as Qi,) he provides a highly suggestible environment where it becomes possible for someone with any kind of listening skills to discern the generalities (and with experience, the specifics) of a condition, then induce a hypno-physiotherapeutic trance of sorts through reverse PP reading.
None of this is magical. Effective application of it requires study (not necessarily in a Scientific context, although I would recommend that approach) and practice.
Edit: The highly suggestible clients probably end up adding their own "flair" to the effects.