That would be the general method to give an opportunity to anyone who is willing to play that role, is a somewhat established member, and can prove though the designed process (and history) that he can detect meritable/undermerited post with a fair criteria.

Nevertheless, most of the merit sources were (I think) appointed without explicitly applying, by means of direct assignment on behalf of @theymos, based on his criteria for suitable candidates, possibly with the aid of some data at some point to pinpoint candidates in specific boards.