ISDB-T closed captioning
Closed captioning (subtitles) seems to be not implemented for ISDB-T. The standard used for it (Japan ARIB STD-B24) is different from DVB/ATSC, so probably new code would be necessary in Tvheadend. The following article has an overview of the format of the closed caption TS streams:
If you decide to go ahead with this, I can supply sample TSs, let me know.