At say 50 kts and full flaps in a C172, you sit comfortably well above a known stall speed
Well, you sit ten knots faster than the POH (172S) speed, so well above...? Above, okay.
I am wondering if you could call it "approved". If so, by whom?
Firstly, the POH (Flight Manual) is FAA approved, and describes the techniques and speeds for flying the airplane. If something you're doing in the plane is different than what the POH says, it's not approved.
I'm less expert at who approves training curriculums, but I'm certain that the national authority does - they won't issue a license to a candidate who has not been recommended against the training standards. Instructors must train students to the curriculum, and not in contravention of the airplane POH. But that's the legality, it's the physics of power idle/off flight in power planes which is more important:
Again, referencing the 172S, yes, you can sit high on the approach, full flaps, descending under control at 50 KIAS, with power idle (or a failed engine) so power is not being considered any more. Cessna tells you that for that phase of flight, you should be flying at 65 KIAS (incidentally, Cessna also tells us that "maximum glide" will be achieved at 68 KIAS - flaps up, I'm sure). I don't have information for full flaps glide, but I'm sure we'll all agree that it's much less distance per altitude than flaps up. So you're going down more steeply, and at a slower airspeed than the POH values. That's okay, as long as you can recover it to a zero rate of descent when you need to. It is a pilot's ultimate goal to arrive back to earth at a zero rate of descent! As you are descending (at a steady rate, we'll presume), you are going to have to accelerate upward from that descent path to momentarily arrest your rate of descent to zero (to prevent impact). Accelerating takes energy, and all the energy you have to work with is airspeed only, as the engine is not available to you for power. So Cessna tells you you should be at 65 KIAS, but you've chosen to fly at 50 KIAS, and stall is at 40 KIAS. When you see the ground getting really close, you're going to pull, to achieve that zero rate of descent which will allow the plane to be reused. Cessna has demonstrated that from 65 KIAS, a pilot of average skill can exchange that 25 knot excess airpseed into a zero RoD before the stall. Oh, by the way, from that steeper descent angle, it will require a greater acceleration upward (G) than normal to flare. Slightly greater G requirement means that stall speed goes up when you pull. Cessna test pilots have learned the same lesson I've learned during flight testing, that the ten knot excess speed above stall on approach is just not enough.
Or... you glide the proper speed as per the POH. If you're too high/fast, slip. You can slip at any speed, modulate it from a little slip to more, and recover in a second, with no change in speed. And, if you're wanting to add drag to get down faster, fly a higher airspeed, drag increases as a square of the speed. And, sideslips are approved in the flight manual, and the training material.
Flying slow approaches, or simply flying slowly at low altitude should make your Spidey senses tingle. Some of the scariest flight testing I have done has been demonstrating a landing from a sudden power loss at 50 feet (a design requirement) from speeds slower than "normal" for the plane (Vy). If you want to safely prove this to yourself, do the following: Climb up into your practice area, at least as high as you'd practice stalls from. Choose a "hard deck" altitude several thousand feet up - a round number altitude will make it easier. Stabilize a slow cruise flight 150 feet higher than that hard deck altitude, at the glide speed and flap setting you'd like to evaluate. Close the throttle rapidly, and enter a glide at your proposed speed. As you approach the hard deck as indicated on your altimeter, pull to arrest your descent momentarily at that altitude, prepared to recover a gentle stall there if it does. Were you able to pause the altimeter pointer at that altitude? If so, it's probably a good speed. If you pulled and stalled, dropping through that altitude, it was too slow, you did not retain enough reserve energy in the plane, to spend arresting the descent before you hit.
From observations I have made during testing, I believe that Cessna's lawyers had a role in choosing the climb and glide speeds for their airplanes - probably Cessna's lawyers have learned the hard way!