I've been pondering this for a moment and if I were to write an application for such a system I don't think I'd enjoy it. If you want to pass information from the kernel and bubble that all the way to the application to do something with it, I would think you now have to write signal handling in your application.
Obviously, the signal handling code can live in a library of some sort.
With a generic, system wide api there’s no reason you couldn’t still poll if you really wanted to. But maybe at least with a version token. So the application can say “I want info on which processes are running. Last time I asked, you gave me token 5827275” and the kernel can respond and say “you’ve got the most recent version of that data already champ”.
