FFmpeg doesn't support doing that as-is and needs certain modifications/additions to allow that, which we've been working on.
Printable View
Just an update - Mezzmo v2.6.4 has been released and it now streams external subtitles to your WD TV Live/Plus/Hub.
Please send us a set of logs when you stream the .ts file to your WD TV Live Plus and we'll see what may be going wrong.