Grant makes a good point about the type of attachment, especially an external piece of equipment. Generally speaking if it's clamped on without backing out existing screws in order to mount it then it will probably fly without FSDO approval.
I recently installed two external antennas for live video downlink in the area where the front gear faring meets the fuselage using stainless hose clamps and then safety wiring the screws. I had my mechanic look at it a he said it would meet the "not permanent" test. I was more concerned with having to drill permanent holes in the aircraft than the FSDO having to approve it.
In the case of mounting a camera on the end of the horizontal stabilizer, it may be more difficult to get a sign off due to the cables running through the tail cone. It would be really nice to have a rear mounted external camera but, for now I'm going to have to be happy with a Sony XC-777 clamped to the center top of the rear cabin looking out the front windscreen.
