When I've created challenge caches I've done the hard work manually. Tried to find people in my area with the most finds/most of one type or whatever it is, and then surfed their profile to see if they qualify. If they do, I put the profile name in a reviewer note till I have the amount of people needed in my area - or if many qualify I've put "at least xxx people are qualified according to (pgc-link to a stat-page". I haven't had any trouble with the reviewers doing so - even when I've sent it in and not yet been qualified myself, but promised to be before the release. (I own 27 challenge caches so far).