"I do not think an exam, length of membership, or number of posts should be the basic criterion for evaluator status . The moderators Know who are the proper members to undertake this. If a member indicates willingness to participate the moderators can decide on suitability.
This would establish a pool of qualified reviewers."

And wind up with a load of complaints from those who are not chosen. If people take a test and fail they can't complain - they can either sulk or improve.

"All pool members coud review any code but I suspect that those best suited to the task are also tose with the least free time."

This would definitely be a big problem.

"I feel for Craig and Steve asthis site grows there will be more work thrust on them."

Which is another reason why I think the system should be automated.