[aerohead]

In the past, 'shrouding' trailers have been used to isolate R-R from overall drag.
Inside the trailer, the test vehicle is completely shrouded and impervious to aerodynamic forces, other than wheel windage.
The car is anchored to the tow vehicle via a tow-bar, which incorporates a linear strain-gauge load cell, designed to allow calculation of the rolling-resistance force.
Locked in place until up to test velocity, the tow-bar mechanism is unlocked for the duration of the test, then re-locked once data acquisition is completed.
Afterwards, this data can be used in conjunction with SAE-approved coastdown results, to help isolate strictly aerodynamic forces.
Amateurs could use a 'fish'-scale of appropriate range for lower resolution measurements.