I'm not sure of the purpose of joe's quest here... do we just want consumers to have a consumer-reports like resource to judge the effectiveness of one teacher vs. another? 
Or is it more like an accountability thing... where the teacher is giving a service and whatever you're paying for can't be picked up and held in your hand... so it should be quantified some other way so that people know that when they fork out the cash, they're getting something measurable in exchange?
Joe, if you're saying that you expect to see "such and such student went on to win this and that tournament... that's already in place for a lot of instructors. Look at this bert kinister page for example: 
http://www.seyberts.com/instructional/videos/bert/videos.htm
I don't think a formal system of following up on students, measuring progress, and publicly reporting the results can be done in a fair and accurate way, for reasons others have mentioned, plus it sounds like a bureaucratic hassle.
 - it's a pain to judge your improvement all that precisely. Three months later you miss less and win more, but nobody really wants to keep track of missable balls made, shape missed, games won or lost, or whatever... a player may know for a certainty he plays better but can't quantify it.
- Some students only learn to beat their buddies or to gamble better, they have no tournament results to point to, and they can't publicly post their gambling successes, so again it's hard to quantify.
- An instructor can be perfectly good but get a streak of students who simply lack motivation or are willing to settle for being mediocre.
- By the same token, an instructor could be just decent, but lucks into a naturally talented, near-pro level student (niels feijen?) and get more credit and business than he deserves.
I think this is probably an ain't broke/don't fix it situation, website efforts notwithstanding. The market is taking care of this one, the rep and word of mouth and referrals are bringing the cream to the top. If anyone is still leery, they can just fork out the cash for instructors who are also proven excellent players - i.e. hire tony or earl or allison.