Trying to get any bureaucrat to give you anything in writing that allows you to do something is next to impossible.
That's just not true. For example, I had no trouble at all getting field approval to mount a Steelebrook EasyLight system on my Tiger. Yes, it took me an hour or two to address all the issues required for a field approval of installation of the device (which had no PMA or STC), but it was quite straightforward and rather easy to accomplish. Where people seem to have problems is when they want to do stuff that has the potential to change the airplane's aerodynamics or performance, or increase loads on systems, and they want that approved by the waving of a magic wand without going through the testing and evaluation necessary to confirm the result will still meet FAA standards.
For example, you want to hang a camera out the airplane's exterior. What effect will that have on the airflows over the lifting and control surfaces? "Oh, it's just a little camera -- not much if any." Well, how do you know that? How little is "little"? What data do you have to support your hypothesis? How about a camera that is just a little bigger than "little"? Or bigger than that? Or bigger still?
The FAA, in approving your installation, is certifying that your altered airplane will still handle and perform to FAA certification standards. If you can't prove that to them, they're not going to approve it. Yes, that is expensive and time-consuming, and that's one reason production-certified planes cost twice as much as they would if they were Experimental-Amateur Built. But if you don't want to go the E-AB route, that's the price you pay.
