I can't find anything on this online. It would seem to be a useful technique as it would lower the delta-v for the vehicle that lifts the payload from the ground to the rendezvous point relative to fully making orbit. (NOTE: Yes, you'd have to get extra fuel to the tug to perform this manoeuver but that's potentially a smaller problem.)
This is the opposite ...
Is this theoretically correct? Is this practically feasible?
I get the same result using the rocket equation, yes.
The tug described can move a 47 ton payload through that Hohmann transfer; it will then be completely out of fuel and unable to maneuver anywhere else.