How can ffmpeg extract dvd_subtitles to separate file?

How can I extract dvd_subtitles from an .mkv file using ffmpeg into a separate file like a .sub or .idx? Or both?

Thank you for your help/advice.

Does not appear to be possible yet:
FFmpeg Bug Tracker: #2391 - VobSub muxer.

For now all you can do with ffmpeg is re-mux to MKV or VOB:

ffmpeg -i input.vob -map 0:s -c copy output.mkv