Both the 140 chassis and the 124 chassis cars use solid state PWM blower motor controllers AKA "porcupine". So there would not be a power issue as it relates to the blower motor itself. I would suspect that the potentiometer in this application is simply a control signal for the CCU processor, the signal is an input and it is at some point compared to one or more other inputs, likely sensors. The CCU processor uses these various inputs to determine necessary output controls i.e. blower motor speed, monovalve frequency, AC compressor clutch, etc.

If the application in question 87 124 blower motor control updated to a stand alone infinitely variable speed control is the goal there are many PWM DC motor controls that should be available to do that. Disconnect the CCU control wire at the three pole bulkhead connector and connect the stand alone controller output in its place. Adjust the blower motor speed via a control pot located in the cabin.

The original question seems to assume a resistance based voltage di

If the desire is to integrate such a function into the stock controls while maintaining stock form and function it would be very difficult/expensive to do so.

I am somewhat surprised that someone in the aftermarket industry has not yet developed a completely programmable microprocessor CCU box to replace the 20+ year old OEM controls. This is the perfect application for the modern micro controller, reading sensor states, processing according to an algorithm, and then outputting states to necessary driver circuits as needed. I suppose the development and manufacturing costs can't be supported by the limited market demand.

It appears the original question diagram illustrates a resistance based voltage divider speed control, this is completely impracticle. Even 123/early 126 chassis cars use a switched resistor ladder controller because a high power pot would have been prohibitively expensive.