A couple of months ago I suggested something similar, but for the Bounty section (see Where to run a spam free bounty campaign? [new board?]). The idea was to use the earned merit in a 48 hour period to establish the sort order, but I did complicate the idea by allowing the screen to have a section ordered by the merit criteria, and a section of the screen ordered by the conventional last post criteria (in order to allow both criterias to coexist and not give full power to merits).