in a campaign i was attempting to indicate a enemy token so the gm would know which enemy i was shooting at. so i got out the freehand drawing tool and drew on the token. well after i could not erase the marking and the gm i assume could have but did not. it would be nice to allow the player who drew something to also be able to erase it after.
also this is not the most effective method of marking/indicating targets. the ping method would work great, but could understand why you have not enabled that for players. (if it is anything like gw then there are sure to be ping-happy players that just do it because they can.) it would be nice for players to have someway of marking/indicating things to the gm as a player, especially if the things we want to mark are not controllable by the player.
an idea from GW is being able to draw that disappears after several seconds. that would be hard to code though i bet. the ping really would work great. and maybe the gm could just ping-ban players who will not use it with discretion.