I've just created the script using newly implemented PGC_GetNumCachesInArea function that gives us the opportunity to create checkers for "Find at least XX% of caches available in Country/Region/Area" challenges. Here comes the script: http://project-gc.com/Tools/Challenges?edit&scriptId=269
I hope that the script will work properly. I'm going to develop and improve it in the next few days.
Please let me know if you have any comments on this :)
PS. I'm also not sure if the script name and tags are enough good and descriptive... what's your opinion?