You are correct about the transport valve. In addition, the transport valve allowed the oil to return to the sump when the load lessened.
The maximum lift on the lift arms could be adjusted with shims in the transport valve. By adding shims more weight transfer was obtained before the transport valve bypassed oil to the remote cylinder which would lessen the load by raising the implement on it's transport wheels. The valve control knob, when turned the other way, also locked the flow of oil from returning to the sump for transporting the implement down the road.
Without using the lift arms you can only lessen the load by raising the implement with it's transport wheels. This can be done automatically by by the traction boost when the lift arms are all the way up and then oil is sent to the remote cylinder.
We did have a traction boost hitch for a 190XT that consisted of a hitch that connected to the lift arms (three point)and to the drawbar. It was two plates about 3" apart (one on top of the other) that had a pin hole inset several inches for pulling pull type implements. As I remember, the implement hitch had to be shimed, on the pin, to make it fit tight in the boost hitch so when raised it tried to lift the whole implement like it was fully mounted thereby transfering weight. I also remember it was very heavy to install and dirty because, like a fifth wheel on a semi-truck, it had to be greased to reduce wear when turning.