I think you're looking at this backwards. Instead of an API, to start you allow users to send or forward their notification emails to some unique email address that's their feed. RSS notifications are handled similarly by the user subscribing to their feeds within the app. Tie into the existing notification mechanisms instead of asking the world to change just for you.

Given the amount of reaction this comment thread got, I'm considering hacking out a prototype and seeing if it helps alleviate the problem. That being said, I've already got far too much on my plate. I don't think you'd ever come up with this idea if you were the kind of person who had time to kill checking their notifications.