THC really does display teletext subtitles, and even allows you to record them inside a .ts file as long as TS has been selected as format in recording settings, and subtitles as being included in it. Playing them back is possible using VLC by selecting Video > Subtitles Track while playing – I’ve yet to find out how to change the default from not displaying them to displaying the first or any subtitle track found in the stream.
I’m was also looking for a way to somehow demux the subtitles, or better yet, convert them into one of the text file formats typically used with MPEG4-compressed video files, and came across ProjectX, the holy grail of demuxers. Once I found the right teletext page, I was able to demux the subtitles – and there’s a whole list of subtitle output formats to choose from.