aircrash, if you’re comfortable working with the Query module, there may be a fairly straight-forward solution (I haven’t verified this).
The unsubscription links are unique to each individual email address via an “Authcode”. The URL for unsubscription links in the current build of EE is:
<a href="http://www.domain.com/index.php?ACT=5&id=AUTHCODE">http://www.domain.com/index.php?ACT=5&id=AUTHCODE</a>
The Authcodes are stored in the “exp_mailing_list” table in the DB in the same row as the user’s email address.
So if the Notify extension processes the “normal” template prior to sending it out, you should be able to use the Query module to do a cross-reference with the “exp_mailing_list” database, matching the email to the Authcode in order to build your link. Something like this (perhaps in an embedded template if necessary):
{exp:query sql="SELECT authcode FROM exp_mailing_list WHERE email = '{email}'"}
<a href="http://www.domain.com/index.php?ACT=5&id={authcode}">http://www.domain.com/index.php?ACT=5&id;={authcode}</a>
{/exp:query}