I'm not sure if anyone here had the same idea, but it's wrong. All you have to do is go to your "edit profile" page (after logging into LJ with OpenID), scroll down to "Contact Info", and put in the e-mail that you want notifications sent to. You'll get an e-mail shortly after asking you to verify it, and then it'll work just as if you had an account there... except you don't.
I've been using my OpenID account for a while to read my friends' journals on LJ, and it's been working pretty well for me. In fact yesterday I finally decided to delete my LJ and so far I don't feel like reactivating it.
So if you know anyone who's still waffling between making a permanent move here because all their friends are still at LJ, then you should tell them about this... OpenID is like methadone. :)