Your idea works well for other kinds of more abstract critical sections too, such as code regions where a garbage collection should be temporarily deferred.
Or in embedded systems where code is running at supervisor level, regions where an interrupt should be deferred, thereby allowing interrupt-disable and interrupt-enable instructions to be avoided.
An alternative which is reasonably fast on modern CPUs is to have a spinlock implementation in the kernel VDSO (or equivalent).
Something a bit like that was done in uCLinux for old ARMs, except instead of a spinlock, they put atomic compare-and-swap in the VDSO, as the CPU didn't have an instruction for that. The kernel checked the userspace address on interrupts, and altered the PC register to redo the compare if the sequence was interrupted.
So that didn't extend the timeslice to allow the operation to complete. Instead it unwound the operation to ensure correct semantics. With a spinlock/mutex combination, cutting short the spinning phase when the task is descheduled by the kernel would be similar, and would allow the "normal" spin count to be unlimited.
