From the last RfC meeting on 13 August 2014:
21:27:49 <sumanah> brion: Followup from 16 July: does Brion know the next steps re CSSJanus on vertical writing support?
21:27:58 <sumanah> https://www.mediawiki.org/wiki/Architecture_meetings/RFC_review_2014-07-16 and https://www.mediawiki.org/wiki/Requests_for_comment/Vertical_writing_support
21:28:23 <brion> sumanah: if nobody else wants to take it it’s still on my research todo list, but i have to follow up on it further
21:29:50 <sumanah> brion: well it was assigned to you which is why I think people thought it was your task
21:29:57 <brion> yes :)
21:29:59 <sumanah> :)
21:30:15 <brion> just haven’t gotten to it yet, it’s still in my queue
21:30:18 <sumanah> got it
21:30:30 <sumanah> np, if the RfC authors want more activity happening, they have their own TODOs
I don't think we know for sure. Certainly, different editors will work differently (VE seems to work, at least in stand-alone mode, but I'm not at all an expert in or even a speaker of any TTB languages). Once that lands, someone will need to do an audit of pretty much every feature to decide if we good enough to declare it something MediaWiki supports, much less Wikimedia does and create a TTB language's wiki.