Angle of attack is the angle between the direction in which the wing is moving relative (relative wind) to the chord line of the wing. It bears no relation to the horizon. It can be low in a climb and high in a glide depending on the elevator position. During my full-scale aerobatic training, I would occasionally pull too hard at the top of a loop and stall the wing while the airplane was inverted.
The two holes in the ball would be located as shown. At zero AOA, pressure would be equal at both holes. At a positive AOA, the lower hole would have a greater pressure and the upper one would have less. It would be possible to calculate the difference but I suspect that for any real accuracy, it must be calibrated with a measured angle. For model speeds, the pressure difference would quite small and very good accuracy would be required to give meaningful results.
Gremlin, I think I learned in a 232. I remember doing loops one day and when I came down an instructor said "I don't think I would do that!" I asked why, he said that plane wasn't stressed to fly upside down.
Your instuctor didn’t have his thinking cap on that day. In a loop the g-forces are always positive. The fact that the aircraft is upside down is not relevant. During a loop the stress at that point is no more than in normal level flight.