Just my observations (worth what you paid for them),
I am an instructor in a very high risk profession, and with the training we conduct we always try to have an outside evaluator do the final check. We use an hierarchy of choices, starting from highest up and furthest from our organization and ending at individual instructors swapping students as a worst case.... critical point being that you don't get your eval from the one who did the instructing.
I like Eric's idea of isolating the instruction from the evaluation in scuba, largely due to the human tendency to not see their own faults (if I suck at teaching buoyancy, per se, and am checking my own students.... how good does their buoyancy have to be for me to pass them?). This would also give the instructors better feedback (Joe Snuffy failed his check dive due to _____. This is the third student from that instructor with that failure, is it the student or a weakness in the instruction?). It doesn't have to be a "higher governing body," it could be as simple as an agreement between 2 shops (If I can, I'll check your students, and you check mine), and swapping students within the shop if an outside instructor isn't available.
Secondarily to this, by separating the check dives from the instruction the source of instruction ceases to matter. Pay for the check dive separately, and it's up to instructors (and/or shops) to determine price of the instruction piece. If Joe Snuffy comes in cold and says " I want to take my check dives," and is apprised of the fact that he CAN fail them and have to pay to try again, then it doesn't matter if he was taught by your shop, a different shop, or the school of YouTube. Either they pass and get their card, fail with a detailed explanation of which standard(s) they failed, or fail with a major safety violation and an explanation of why the Eval was cut short (for their safety).
I do think this would open the door to more budget limited divers, and not prevent those who can afford more of the shops time from using it as well. It would also most likely result in an improvement of the instructional community (I know that those who evaluate my students in my profession always give me feedback, good or bad, on how they did and we share teaching techniques more because of it).
Respectfully,
James