A job for Friend & Ignore Share, to be specific. F&IS is a low-configuration, drop-in solution to my problem. After it's installed, your friends lists will automatically be synchronized between all your characters. Also, by default, your alts are all added to your friends lists for easier mailing auto-complete. Ignore lists are also synchronized, as the name suggests, although this is responsible for some "player not found" errors at log-in (players need to be online to be added to the ignore list).