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).
It does have a few issues. First, the game puts a limit of 50 on the friends list; therefore, if you have more than 50 friends across your characters, you'll get an error. This is not really the mod's fault, but it is slightly irritating. Second, note-synching is not automatic. If you want to synch notes, you have to log into the character with the notes and do "/fs reset," and then do "/fs notes" on all the characters you want to import the notes to. Still, it's a relatively small price to pay for a big increase in convenience (for me, anyway).
Does anyone know of any better friends mods out there? My dream mod would do all that F&IS does, along with automatic note synching and no limit on the size of the friends list. If I could categorize friends too (tanks, healers, DPS), that would be ideal.
Download Friend & Ignore Share at WoWUI.