I have done this on a few wagons, by motorising the wagon correct loose shunting can be performed. The only difficulty is moving the loco and wagons at the same time - needs an extra operator! It was shown at a few exhibitions in the past.
In the case of your description of the coach, it would probably be easier, as when the coach is gravitating back, the loco would be stationary. You could have on-board uncouplers as well - a real 'hands-off' experience.
A motor in the coach could replicate the mass of the coach, it would need to move at about walking pace. When in the train with the loco, it would run as a consist.