Sounds marvelous. One thing that I especially like about it is that I don’t see any reason why it couldn’t be implemented by LastPass, and they have a lovely bookmarklet interface that means they’re portable to just about any browser.
Have you considered asking LastPass if they’d be interested in prototyping a test implementation of something like this? They’d seem to be ideal since they already have the secure-password-management-in-the-cloud side of things taken care of, and users who would presumably be very receptive to even more automation. You could implement the provider part on Stack Exchange to give it some production usage.