Well, I discussed this wth a friend recently, concerning halo 4 and such. The covenant only wanted to fire the rings because of their "Great Journey". Now that the religious leader (Truth) is gone, im sure that the covenant will be in dissarray and spli into factions. I believe they wont have the man-power to accomplish such a task. But it is still possible if the indexes were found. But, until halo 4 comes out, we can only speculate.theoretically, if they have a human prisoner to activate it for them, yes. but its been stated following the death of Truth, the covenant fell into disarray. There's no reason for them to activate the rings without the prophets leadership.