We (Dutch Moodle users at our Moot last week) happen to have discussed possibilities for this. If you mean rating competencies in the same way a teacher would do for a student, but more in a peer assessment kind of way, you could try one of the custom roles for this.
A huge note of caution though: in peer assessment it's most useful to have at least 3 assessments per user, to have a nicely balanced score/rating. With the current set up of reviewing and rating competencies, this can not be done: 1 rating per competency. This probably has to do with the nature of CBL/CBE and the preferred 'competent/not yet competent' scale for this. In such a black-and-white situation, there is no place for multiple assessments/ratings for any given competency: you're either competent or you're not.