The really interesting bit is at the end: it turns out Pixiv actually released their player as open source! It's available at https://github.com/pixiv/zip_player. It looks like it would be pretty feasible to integrate this into Danbooru to give us native ugoira support.
It's not hard to implement anyway, using external libraries it only took me around 70LoC.
The real issue is that it it only works in the browser, you can't really save it to disk in animated form or post it anywhere else on the web.
The zip + frame times is good for archival purposes, but not convenient otherwise.