A Very Good Question
I would expect all cachers taking part in this quest to act responsibly, and keep any code numbers they have found confidential, just as they would for any other puzzle cache. Also as in Draft Guideline 9, the cache owner would be expected to monitor log postings for "Spoilers".
Anyway what good does it do to release the code numbers to others? It may mean that they finish the series before you do!
Because the code numbers do not directly lead to the final co-ordinates it would be possible to issue new numbers to caches if the codes for them become common knowledge. That would then mean modifying the records of anyone who had already found the cache legitimately. However this might not be a trivial task, it depends how we manage to keep the records. If everyone who logs all their caches as they find them, and then either separately logs them with Big Quest, (or if some way can be found to extract that info from the Geocaching site) it should be fairly easy to update their records with the new code value and/or email the cachers with the replacement code.
I had hoped that once all the codes for a region / Country/ British Isles had been found the cacher could simply enter the code on the BigQust site, or email the site, and have the co-ordinates sent to them. probably after a check that they had actually logged all the sites. However this is currently against the Groundspeak rules. They don't normally allow real cache co-ordinates to be issued by any other system except their site. But they may allow it in this case (Heres hoping). The DeLorme series of caches in the USA allow the owner of the "final" cache to supply real co-ordinates to a legitimate cacher, after their logs for claimed finds are checked. This is very similar to what we want to do, so there is a chance that we will also be allowed to do it.
I am in he process of trying to put together a e-letter to them to see if they would give their approval. One thing I think they will need to be sure of, before agreeing to the request, is that this is a serious project. But, judging by the interest shown on this board I don't think that that will be difficult.
Failing getting that approval I am working on a system, which I think will be fairly secure, where the co-ordinates for "final" cache are "Hidden in Plain View" within a largish number grid, published with on the final cache specification page. More on that later, if it is required.
Re cheat proof: What about a photo of the cacher at each location as in the American version.
This would prevent people signing on their mates behalf, so they can cover differant regions quickly.