Allow users from a given statusnet instance to interact with another instance as themselves

There are occasions where it would be extremely convenient to interact with a user their own statusnet instance (that you don't have an account on), but you follow them on your own instance?

For example, currently:

User A on Instance X follows User B on Instance Y.
User A clicks through to User B's profile on Instance Y.
User A wants to comment on a post that User B made.
User A has to go back to Instance X and trawl through their own timeline to find where User B's post exists before they can post on it.

We already have the ability to log into identi.ca (for example) with OpenID, but that just logs you into identi.ca as an identi.ca user.

My suggestion is to allow User A to log on to Instance Y using their URL (e.g. http://instancex.com/usera), be authenticated through Instance X, and then be able to comment on posts made on Instance Y using their *own* credentials.