You could try looking on the DS parking page for a "for sale" or "contact the owner of this domain" link. Sometimes these links aren't exactly prominent so you might have overlooked them.
Another option would be to get yourself a domaintools.com membership and check previous whois records, until you find one with an email address listed.
If all this fails... Report the domain for having false/incorrect whois information. The registrant will have to update whois info then, or risk losing the domain...
Whois Data Problem Report
Hope this helps
- Rob