SS,
I appreciate that you are probably refering to the FAA BFR, but I believe the JAA one requires a 1 hour training flight with an instructor. In that case I can't see any way to see that being anything other than pilot under training. Otherwise it wouldn't be a training flight.
In the circumstance described, I think I would be agreeing with the instructor before the flight that I would have the authority to end the training flight and take over as pilot in command at any point in the flight, and that he wouldn't object to that. I don't think too many would have a problem with that, but none the less, it would be best to have that sorted out before take off.
dp