There is a way....but it isn't as clean as you'd like.
1. Use the Create Short URL action to create a http://miurl.cc link for their portal. You can either do this using Method actions (and store this URL with your own credentials in your current web site database), or dynamically using the Method API (you need to call our web services using code).
2. Append to the end of the portal link the contact's email address.
So, if the URL you generate is http://miurl.cc/abc123 then append ?e=email@email.com, to get a final URL of http://miurl.cc/abc123?e=email@email.com
Redirect to that URL and you'll get single sign on.
Sorry.....not that clean....but not as bad as it looks.
Paul