You most likely already have a tight loop lying around - the compositor needs to composite the screen each frame, you can probably poll in there. The client likely has one too, if not, you can involve the kernel & scheduler. If you need super high precision you probably busy wait, I don't know what the Linux scheduler's resolution is.

I would probably expose a poll() and let the client deal with it, I don't know if there's a one-size-fits-all signaling mechanism. But you have control over it, which is probably another plus.

Those loops aren’t tight, they sleep for like 10-16ms for live windows or much longer for background windows. You were talking about receiving updates at 1000Hz.