I think the only possible work around then would be manual intervention.

You would have to not allow manual sign ups for the forum, they would have to be admin created or script created.

Then send the user an email saying they can login to the forums with the same login/password they have already set up and give them an option to turn on a cookie to keep them logged in.

Obviously it doesn't scale to the 100,000 user mark very well but if you could batch automate the forum account creation based on information pulled from VShare that might work out nicely. As opposed to ripping all the code apart anyway.

I'd love to hear any of your thoughts on this workaround!