I suspect this wouldn't happen more than once a week, and even that would be a high estimate. Is it worth the coding time? I guess you probably already know how many signup attempts get bounced per day.
Perhaps it could just be a PM from the Member to you or an admin saying "please create an account for my friend, who has the email address
haxorrdude@aol.com, and give him the username
haxorrdude"?
I think this could work and still prevent the "Bones" problem.