Okay, here's my idea:
Watari can post as L and as Watari ONCE A WEEK. Once all the Ls die, the watari becomes an L and loses all of his features as a watari. If a watari dies before an L, then the L's can only post twice a week. Also, watari's know who all the L's are, but isn't allowed to tell the L's who the other L's are.