In terms of other people turning a transmitter on in the pits, this is a problem that has been faced by RC since it began. Before 2.4 GHz is was routine for big RC car races to have a transmitter impound. You got your transmitter back just before your heat.
Big difference this time is that the VEHICLE is the transmitter (video coming back to the operator). "Impounding" vehicles is a non-starter... folks have to work on/repair their aircraft between races. Props break, wires get sliced, PCB's and/or their components get dinged... it's a lot like an airborne demolition derby. Impound aircraft and the sport will cease to exist overnight.
And no, we cannot "just impound the video transmitter PCB". To keep size and cross sectional area to a minimum, the PCB "stack" is a tight assembly fully customized by each pilot. You can't just add or remove PCB's as a matter of convenience... even repairs can involve almost complete diassembly and reassembly depending upon how the aircraft is laid out. This problem needs to be solved with technology, not social engineering.
EDIT: Here's an example. At the World Championships in Shenzhen, DJI (the "stable camera platform" drone company) provided HD cameras with integrated video transmitters so the organizers could livestream high quality video from each aircraft's perspective during each race. Those little cameras were expensive, and they only had so many, so DJI staff were standing at the flight line to install them on each aircraft as they were carried out to start a heat and then they recovered them off each aircraft when they were carried back after each heat. They had a variety of ways to attach them - zipties, double sided foam tape, you name it - but the chaos was ridiculous. The pilots got into serious arguments when the DJI people picked where they wanted to mount them... many pilots didn't appreciate the DJI folks "manhandling" their aircraft as they treated the cameras like precious gems... the DJI people got angry each time a camera was damaged from an impact (as if the pilots were doing it on purpose?!?)... etc. This just illustrates the craziness of trying to remove/impound/add some piece of electronics prior to each heat, and in this case it didn't even have to tie into the other electronics on the platform.
There are at least a dozen popular video transmitters in current use in the FPV world. Each optimizes for some different set of features. Sometimes size is important. Other times features win out even if the board gets larger. Some people insist on coax connectors on the board, while others demand to have the coax soldered straight to the PCB. Some Tx's are standalone, while others tie into the flight controller to get overlay/HUD info such as battery voltage, present/peak current draw, elapsed time, etc. so the pilot knows how much is left "in the tank" and whether he has to nurse things along to survive the full heat length. There's no one size fits all, and if DJI's experience is any indication the swapping of hardware in real time isn't an acceptable solution.
the issue I have with FPV is that we could just simulate the entire thing like a video game.
Simulators (aka trainers) do exist. However, I notice that real fixed and rotary wing RC modeling continues to be popular despite incredibly real simulators being available for many years now on a variety of platforms. There's something about "reality". Plus, a huge draw of this sport for a lot of the pilots is the merging of their love of flying with their love of electronics, assembly, craftsmanship, etc. And frankly from this Dad's perspective, I'm a lot more supportive of FPV than I would be if my son were living in the basement playing video games like so many other teens these days. The skills he's building in hardware, software, mechanical, 3D printing, social interaction with sponsors and other pilots, etc. are transferable to a wide range of career options and life lessons. He's considering majoring in Aeronautical Engineering when he starts college next year, which I doubt would have happened if he'd been playing video games.
btw, if you've raced at major drone racing events then odds are very good you've used hardware I've built!
OK, you can't just leave that hanging out there! {grin} Fess up, what are you talking about? (And noting the flag you have in your ID, my son took first place in the Western Regional Canada Championships in Kelowna BC last year and 4th place (top non-Canada pilot) in Canadian Nationals in Ottawa ON a few months later, so if anything you've built is local to Canada the odds are actually quite high!)