I know that I can transfer licenses between Xbox 360 consoles (say, if replacing a broken console), but is there any way to transfer (permanently move) licenses/ownership from one gamertag to another (say, when retiring a gamertag)?
You can not transfer the license of content you purchased to another account. You can, however, set content you've purchased to be accessible by other profiles on another Xbox (possibly what you're already aware of).
See here for details: http://support.microsoft.com/kb/963711
And another link I found: http://support.xbox.com/en-US/xbox-live/marketplace-and-purchasing/download-content