It's a fascinating discussion and I'll come back to it after coffee :) My initial thoughts are that if annabel is "logged in" as dw_annabel, she can still post as fic_annabel. So all we need to do is pass something from the email that says "post_as", that overrides the "logged_in_as", and we're done. Then we just need to check that the logged-in account can post as the post-as account.

But I hadn't thought about email notfications, so I'll have to think about it more.