Welp, turns out there was one last (I hope!) brown paper bag bug for Fedora 20: fedup 0.7, which was in the Fedora 18 and 19 stable repos today, can't successfully upgrade to Fedora 20 any more. It certainly could last time we tested it, so this has blindsided us a bit, or else we'd have been better prepared!
Fedup 0.8, which is currently in updates-testing but will be in stable for Fedora 19 tomorrow and for Fedora 18 just as soon as we can get it pushed, can upgrade to Fedora 20 just fine. If you want to upgrade but haven't got around to it yet, just make sure you use fedup 0.8, and you'll be fine. Run 'yum --enablerepo=updates-testing update fedup' to get 0.8, or you can use the graphical package management tools to enable updates-testing, install fedup, then disable it again.
If you already tried with 0.7 and it failed, just upgrade to 0.8 and try again, but you may want to do 'mv /var/lib/fedora-upgrade /var/lib/system-upgrade' and 'mv /var/tmp/fedora-upgrade /var/tmp/system-upgrade' first (fedup's file download locations changed in 0.8, and this will save it needlessly re-downloading the upgrade packages, and make sure it cleans up after itself properly when it's done).
If you already had a failure with 0.7 and a success with 0.8, you might want to check for /var/lib/fedora-upgrade and /var/tmp/fedora-upgrade and wipe them if they exist - they won't hurt anything, but you don't need them and they're just wasting disk space.
Sorry for the mess, folks!