My recommendation would be to use a plug-in like Lightbox Plus to power your link. Clicking the link would load your standard login form inside a lightbox. You could then automatically redirect to the same page so that, once logged in, users can leave a comment.
Basically, you'd dynamically build your link to be something link this:
title="Please login to comment">Login to Comment</a>
This would load the login form in a lightbox and set it to redirect to the post upon successful authentication. This might be all you need, but I haven't tested it ... so give it a try and, if needed, add some other scripts to reload the page upon successful login.