Adding support for closed captions to the WM Screeners app is a feature we are actively working on as of November 2018. We have it working in our Dev environment and we hope to release this feature to the main app in Q1 of 2019. We don't currently have a timeline for Descriptive Audio support.
The only caveat is that there are two parts to this issue; the first is preparing the media publishing workflows and the apps themselves to support captions. The second, more time-consuming part, is making sure all content has closed captions in the format we require. That is the one that will take some time and our plan is to implement closed captions on any newly published asset on a go-forward basis and then retroactively bring legacy content in to the fold. With 80,000+ assets this could take a while which is why we are focusing on the new content first.
The silver lining is that once closed caption support is added this also makes adding multiple language subtitle tracks much easier. Closed Captions are essentially English Subs.