Good article here about the FAA view:
Camera Mount Common Sense - AVweb Insider Article
Main point:
"Basically, the memo says camera mounts aren't major alterations since they don't appreciably change flight characteristics, performance, weight and balance or basic airworthiness. They also don't represent changes to the aircraft's basic type certification, so no STC is necessary."
It goes on to say that if you get it wrong and something bad happens, the FAA can always use 91.13 (careless and reckless operation) anyway.
The absolutist view that ANY external temporary modification requires approval doesn't really pass the common sense test. What about the vinyl 12" numbers I used to make my plane legal to visit Mexico? What about a sticker from a sponsor?