I think implementing this the way you're doing it, where it changes existing setups on upgrade, is a bad idea, because it will leak the disk space used by the original /tmp and the administrator will have no obvious way to get at those files to delete them (even more so if the "administrator" is just an average home user). It's also quite rude to change existing setups that way.
I think this should be set up in the default fstab for new installations only (if at all). (The reason you give for not using fstab is so it also works for upgrades, but that's exactly the wrong thing to do.)
I'm also worried about the compatibility with many existing applications, and I disagree about the Downloads xdg-user-dir being an appropriate place for temporary downloaded files, as opposed to downloads the user intends to keep. But the decision to overrule the existing fstab instead of honoring it is the biggest issue.
--Kkofler 21:03, 31 March 2012 (UTC)