The concern I would have with this is that the active timer for Castrum and Dalriada are visible to everyone in the instance. While it's not foolproof as some runs take longer (or MUCH longer) than others, it's not too hard to ballpark a completion estimate. If someone could just join up "in progress" then what would stop people from just camping out in the instance and then porting in when the group actually doing the content is at the final boss to basically snipe a completion without doing any work?