Support for UT3 maps has landed, including mostly working map screenshot extraction support.
There are some eccentricities such as lack of LevelInfo, so people got creative with adding author names to things like the player count string in the associated .ini files, so a fair amount of brittle scraping is needed to get those values, but it mostly seems to work.
If anyone has an archive of individually packaged UT3 maps, I can try to bulk import those to get things going. I cannot scrape http://ut3.ut-files.com/index.php?dir=Maps/ because the throttling there seems to have got a bit much, even just browsing the site to find test maps gives me a "Please don't hammer the site" error most of the time.
Otherwise, the submission option on the website works as well if you have smaller collections to upload.
I'll try to figure out mutators next.
Edit: I should also add - there's no "Themes" support. It looks like the only "themes" UT3 really has are "Human", "Necris", and "Liandri Tech". The lowered reliance on BSP also means the method I used to determine themes doesn't really apply well to these maps, so something else to think about for later.