In theory this is a good idea, but it's probably complicated to implement, and questionable how much of a call there is for it. It's hard to imagine someone who doesn't have a DW account choosing to fork over cash for a username on a site which s/he may well end up not using. (I suppose DW admins could find out how many people buy a rename within 24 hours of opening an account... if that turns out to be a large number, this suggestion may be worth implementing.)

Maybe just write a FAQ question explaining that if you want to start an account with a purged username, you should create an account with a gibberish name, then click here to rename.