I'm not one of the gods, so I'm just speaking with speculative friendly advice.

Open up a username that is similar, perhaps with a hyphen suffix or something. Then send a /msg to the gods, indicating your intent to become an active member of the Monastery, and that you have your sights set on that particular username.

Then, be patient, participate, ask, answer, and in general, enjoy frequenting the monastery. Maybe send another petition to the gods after a few weeks, if you still feel the same way, and if you've so far proven to be more than a one-hit-wonder.

Again, I'm speculating, but I wouldn't be surprised if at some point your wish will be granted, again, assuming you participate, and assuming there isn't any activity whatsoever in the form of posts from the old username, and that person hasn't logged on in years.

Good luck. If its any consolation, I spent my first week or so at the Monastery as davido@someemail.address. Fortunately the gods were gracious enough to hear my petition (and my prayers perhaps) and granted a change.