> It would be nice if there was a way to use Intel's debug registers to write a value to some address, which would then interrupt some kernel task

What you have described is literally the syscall mechanism. That's what it is. You perform some register write (via a specific instruction) and an interrupt is taken to the kernel. Maybe you believe that an asynchronous interrupt would cost less than a synchronous interrupt for this particular objective but I'm not sure there's evidence for that claim.