Not so excellent. The spreadsheet you sent me had about 40 names on it. C55AHK mentions about 500 charter members. Many of these are no longer active, and their caches have been adopted by others. GC55AHK allows the claim of these adopted caches. Adopted caches may now be owned by members that are NOT charter members, and so those caches would not be accepted by a challenge checker. And it looks like there are quite a lot like that. That means a challenge checker would be unreliable. Also, although Project-GC might one day export the "is a charter member" status, that also would not solve this adopted problem.
Best method would be to work from the Charter Members Caches bookmark list, but at the moment Project-GC does not collect this information either.
Sorry, schwandt, we have no easy solution here. You are stuck with doing it the hard way.
Tom