I’m also very excited about this idea. The format of the ultimate judgement (i.e. the retrospective evaluation) seems important. A straightforward survey of the population suffers from the problem that teasing out an answer about the quality of a policy is hard, and most people won’t have put the time or effort in (even assuming they don’t have a hidden agenda, as John_Maxwell_IV highlights). But a survey of experts has its own problems too.
That said, I suspect these issues are surmountable, and would be keen to see this idea turn into action.