Want to smoke him/her out? Make a serious offer.
There are myriad ways to connect with a seemingly disconnected domain registrant, but nothing works quite like an offer of serious loot. Money tends to get people's attention.
Email subject line: "Offer $5,000 for Example.com"
Letter: "Offer $5,000 . . .
Phone message: "I'm very interested in . . and I am prepared to pay $5,000 . . .
Etc.
Money talks. Everything else walks.
Post up everything you have done, thus far, besides sending an email to the WhoIs record email.
Before doing that think of all the other ways you might track down the owner - and post those ideas too.
Brainstorm by yourself, post it up and then let's see what else we can suggest.