I cannot guarantee for sure but... for private dummies to be used, player needs to be in Protection Zone.
So, for an example, that house right next to Thais depot (left of depot <- ), I have seen the house owner put his dummy in his door, and someone in the depot training ML, from distance, but not from inside the house, from the depot's protection zone.
Maybe with Hirelings that would work as well, but I have no way to test it right now, so all I can do is speculate...
What I do know is: hireling will not answer to a Neighbour through a wall. Even if that neighbour is invited. Maybe it is because the wall "blocks" the sound and the hireling won't listen, which is a weird mechanism, because we can talk to other NPC from behind the walls.
That makes me think that it is not possible to talk to Hirelings unless you are in his owner's house.