There are people who recover teletext from VHS tapes, for example: http://www.uniquecodeanddata.co.uk/teletext76/bbc1_1983-06-2...
Is there an FFMPEG type of filter that can translate information recorded in the vertical interval space like CCs or even VITC? All of the equipment I ever used expected an incoming video signal for processing. For archival processing, it would be an interesting approach when finding the proper video based equipment would/could be difficult.
Specifically for teletext (so, in general, PAL territories only), you can use this: https://github.com/ali1234/vhs-teletext