I set up a mediawiki.ui test instance on Labs. For now, it is dedicated to showing the in-progress patch which is rolling out mw-ui to all buttons (https://gerrit.wikimedia.org/r/#/c/52169/). You can see what's implemented so far in the commit message in the top right.
The URL is http://mwui.wmflabs.org/wiki/Main_Page . Feel free to check out those pages, create accounts, make test edits, etc. However, it's not a wiki for actual documentation or design mockups, etc.
After that patch is done, the instance can be used for testing other mw.ui work.
Matt Flaschen