Some distinctions here. While commonly referred to as a "ferry permit" it is in fact an Airworthiness Certificate, albeit a special one. It is the legal remedy for operating an aircraft that does not have it's regularly issued Airworthiness Certificate in effect. Extra fuel tanks do not require one unless the aircraft is to be operated over gross weight. This is most often done by STC. Said permit does not allow one to deviate from operating rules, RVSM, MNPS, Part 91, etc. While I'm sure that it happens, those of use in the business and who expect to remain do not go blasting along violating requirements. Waivers and exemptions are "workarounds" for these situations. It can be and is routinely done in a safe and professional manner. Amateurs need not apply. I can cite numerous examples where I was requested to salvage a "ferry" trip gone bad. Civil Aviation Authorities catch many of the renegades-outlaws and ground the aircraft on the spot.