Since this is pretty much already a thing, eg. tribalwarsmaps, i dont see much problem with this implemented. However it shouldnt be based on if you logged in or not, but rather on Points and maybe ODA, which is stats that is pointing towards the player being active or not, but doesnt have to mean the player is inactive.
Actually knowing if a player is logging in or not, for people outside of council of that players tribe, is not a good feature.