Having a smaller breaker to protect the appliance isn’t uncommon with larger appliances on a single run. Your air conditioner is a good example. Older air conditioners used a bunch of energy and had large wires, the newer ones us less energy and specifically spec the max sized breaker for the circuit it is running. Most cases this is less than the older unit. No electrician will run a smaller wire, you just reduce the breaker size.
A little different though because the AC unit is hard wired into the circuit, so the only relevant items are the wires and the AC unit itself. So no problem installing whatever breaker size you want that works with the wiring and of course the appliance itself. But when there is a receptacle involved it becomes a communication issue. A 14-50 outlet effectively announces to the world that this circuit is four-wire 50 amps. Sure there is no real harm if the circuit is only 20 amps, worst case someone unknowingly plugs in a higher power device and trips the circuit breaker, which by the way will make it harder for someone to find the breaker since they will be looking for 50 amp. Of course I realize you will have complete control over what gets plugged into the outlet, and I'm sure you will upgrade the breaker when you sell the house so that it matches the outlet. But codes don't make all of those assumptions. You seem to know what you are doing, and you feel it will actually be a safer installation so I'm not going to challenge or question, but on forums like this I think it's always good to have the discussion, and anyways it's interesting.