[CodeReviewer] A way to rate each participant in a review?

Does anyone know how I might be able to add a way for a Lead reviewer--or, to simplify it, any reviewer--to rate the participation/quality level of each participant in reviews? Like a rating of 1 to 5 for each reviewer: 5 being a valueable contributer, 1 being total lack of participation.  The only way I can think of is to create a Participant Custom Field for *every* user's name.  There would be no way to filter down the number of fields to just the list of users involved in the review though.  :-/