Ultimately, Cyan will have to decide IMHO. Explorers can do technical testing and make sure the age does not break anything. That has to happen in public, without any tester being required to sign up to any formal group. Since actual technical issues are something rather objective, one would quickly find out if so-called issues are just attempts to block an age.
If Cyan actually can not have a rough look at the story of an age itself, I doubt they can actually run and administrate a Shard - uploading and integrating the age also takes some effort.
And one more point, there is a fine line between Cyan saying "this guild is going to decide whether the age abides the IC rules" and "explorers (anyone who wants to, though they might want to organize in a guild-like structure) should sum up the frame of the story in a few sentences, and we will decide whether the age abides our rules". The first is moving of the responsibility to some fan group, the 2nd is having the support of the fans to more easily decide yourself (EDIT: also see Chacal's reply below, basically the same idea).
The whole situation will be much less problematic if Cyan allows completely independent fan-run Shards. In that case, there will be Shards accepting everything technical possible, just like UAM will accept every age and there are people willing to aid in fixing a broken age.