What happens in a preemption? An OS would have to manage this on a per-process level to ensure that a lock doesn't stick around on a preemption. Now the OS has to remember locking info per-process. Also, on resumption of a process, an OS would need a way of locking, but bypassing the lock to set the registers that have been manipulated, without other CPUs interfering.

Your idea is nifty, but it would be a nightmare to implement.