I'm making a new thread for this to make sure everyone is aware of it: Danbooru now has native support for Pixiv's ugoira format. You can upload ugoira by putting one of these links in the source field:
It works using Pixiv's own ugoira player, so animation playback and browser compatibility should be exactly the same as with Pixiv. The original .zip file is stored by Danbooru completely unmodified. The frame metadata is available in Danbooru's JSON API, so existing archiving/conversion tools should be able to be made to work with Danbooru without much trouble.
For downloading purposes, ugoira are automatically converted to .webm using Toks' conversion script. Use the "Save as video" link beneath the ugoira to download the .webm; use the Size link in the sidebar to download the original .zip.