Thought about this for a bit. I'm betting it's used. The deal with brakes is the ratio of the friction applicable by the brake and the lever distance of the compared to the lever arm it's stopping. I'm watching a movie so I'm not going to do the math right now but I think the size of disc rotor you could get on say a mule's knee joint versus the lever arm length of say a lower leg might cause problems either with the size of the rotor.

My thinking is mountain bike discs are 120mm/4.8"-203mm/8" diameter compared to a 29" wheel.

These are meant to stop a 160lb person who is going down a hill. Well Slow to a stop not lock dead in place as the wheel would break free so it's not the full stopping power of a real.

Anyway my gut reaction is it might be difficult in this use case. I think a 8" rotor on a similar lever arm might have trouble fully locking down that joint and would be very large and flimsy (8" thin sheet of metal) for rugged use. Plausible though.

But hey I failed out of mechanical engineering into computer science for a reason so maybe I shouldn't be trusted :)

Sure there are. They add complexity and weight and failure conditions. There's also a difference between lock and place and lock at an exact position.