One solution other sites could adopt is a .com ownership rule. If the username you want, suffixed with .com, is a registered domain name, you would have to ver

One solution other sites could adopt is a .com ownership rule. If the username you want, suffixed with .com, is a registered domain name, you would have to verify the postmaster email address at that domain to prove you own it. (Or upload one of those random HTML files like other domain ownership checks do.) This way you can just own the .com domain name of your preferred username and guarantee you'll get it on sites that implement this.