^Sure, that would be the best hope, of course, and I'd love it if there were some vault that turned out to have every remaining lost episode. I'm just thinking that maybe there's a last-ditch option to be investigated when all else fails, and I'm wondering if anyone involved in Doctor Who restoration has given any thought to tracking down the actual physical tapes and determining if there's a way to extract erased data from them.
Well, the tape numbers are known (most, if not all, of them are listed in the wiping paperwork summarised in Wiped, which is written by one of the Restoration Team, and I'm guessing that when there's no wiping paperwork 'Before 1971' means that 'The programme now on this tape was made in 1971, so....'). So they could certainly be checked, assuming they survive (some of these tapes had already been reused so often that they were falling apart: at least one episode of Tomb of the Cybermen was almost untransmittable as a result). If they do survive, they'll now be at the British Film Institute's archive, as the BBC's 2inch tapes were all donated to there after they were transferred to digital D3 in the 1990s.
Your example of the colour restoration from the B&W prints is an interesting one: the problem of the colour info turning up as interference patterns was known, and the technicians were supposed to use a filter to remove the colour info. But as there weren't enough filters (until late 1969 BBC1 wasn't in colour, and all the relevant film copies date from before summer 1971, so you can understand why Enterprises didn't initially have enough), they got on without it without them.
In other words, people got on with doing the job even if it meant breaking the letter of the rules. So there's a chance that some Who tapes were reused without being comprehensively 'blanked'.
But... you mention how it becomes increasingly impossible to retrieve a usable TV signal a few light years out from Earth. Suspect the chances of retrieving anything usable from a reused tape are equally low (though it does make me think: I have an audio cassette where a 'ghost' of a lost Week Ending episode can be heard under the later recording. Maybe, just maybe, it's salvagable).