In 2004 I tried integrating the RA into OS X via the Input Manager mechanism, a way to load external code into Cocoa apps that'd have access to the contents of any text field, so you could get the text that the user was currently looking at and send it to the RA for queries.

If I recall correctly, I never had enough time to make it very useful, and it wasn't clear how best to get its input sources - Mail.app stored email it could read, but other things like calendar and chats were inaccessible to it.

I did it as part of a generic project for making use of input managers: https://sourceforge.net/p/leverage/code/HEAD/tree/

(NOTE: the RA code isn't there, I think it was never even useful enough for me to share it)

My favorite use of that was the "ISIM", or emacs-style incremental search in text views. That was nice. But of course, loading arbitrary external code wasn't sustainable, and that mechanism went away, along with my ISIM.

I no longer track OS X development, so I'm not sure if there's a new equivalent way to do this stuff. It'd be neat to hear.