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

Note: I submitted a change to the Node.js version of CSSJanus on github a while back, implementing vertical writing support.

In T353#2427687, @Yair_rand wrote:

Note: I submitted a change to the Node.js version of CSSJanus on github a while back, implementing vertical writing support.

Here’s the pull request @Yair_rand submitted to CSSJanus, currently blocked on this pull request.

I'm not sure why we must create another task as #RFC, afaik I found T122942, which is an example that modified a single issue task to a RFC, so we can merge this back to T11436?

How to with Vertical writing and horizontal writing mixed text composing?

In T353#2918840, @Shizhao wrote:

How to with Vertical writing and horizontal writing mixed text composing?

{{MongolUnicode}}

I'd still like to do a research spike on this, but recommend taking it off the TechCom RFC board unless/until there's a major internals change more ready to look at.

In T353#3887913, @brion wrote:

I'd still like to do a research spike on this, but recommend taking it off the TechCom RFC board unless/until there's a major internals change more ready to look at.

The "do this" task is the parent, T11436: Vertical writing support in MediaWiki – this is the "discuss and decide how to technically implement this" task… :-)

Declining as an RFC, since this is not proposing a technical solution. But it's certainly interestingly as a problem statement.

In case anyone wants to work on this in the future, what is needed in addition to "Adapt CSSJanus to provide support for CSS 3 vertical text" to be considered a technical solution?

In T353#4160135, @Nikerabbit wrote:

In case anyone wants to work on this in the future, what is needed in addition to "Adapt CSSJanus to provide support for CSS 3 vertical text" to be considered a technical solution?

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.