Tried to add comment there, but it says it needs moderator approval already for 2 hours. So my point is that a) the user him/herself should be able to run the checker notwithstanding the privacy settings; AND b) if the privacy is really a concern, the challenge checkers should be rewritten so that they do not return unnecessary info - only the relevant for the challenge - if the count is required, only count it is, not all the caches found with all the dates, etc.
However I understand that this might require re-writing almost all the challenge checkers and might not be such a good idea.
Then a workaround from GC or PGC side, would be to allow user to check for himself. Then at least user can post a proof in log or PM, that she or he fulfills the challenge.