Hi KiwiME. Your thought process is very similar to mine. However, in the preliminary dyno testing that zenourn was referring to the dyno wasn't in constant speed mode, it was in constant load just like you suggested but it still didn't play nicely with cruise control. Also, all S models and some others don't have cruise control, so this issue has been resolved with a mechanical adjustable foot which is fairly basic but works very nicely.
Regarding measuring the HV battery current at a set speed (on a flat road with no wind) to account for the internal current sensor calibration error between cars, that was also tried but found to be less than ideal. You think a road is dead flat until you get quite different readings repeatedly in each direction :) You could average that as you suggested, but it still didn't seem like the best option.
Our latest method involves tapping into the HV battery with a series wire loop in a safely modified service disconnect switch and a calibrated current clamp. Controlled discharge is being done at C/3 as per the current AESC spec (manufacturer of the cells). Further testing today is looking really promising. We're making a batch of the modified service disconnect switches and planning to make our test method freely available so that others can test in the same way and get comparable results. However, before doing this we want to share our draft of the test procedure with interested peers for their review. We've got a list of obvious people from around NZ including Ruki on this forum, but if you would like to be included then please email firstname.lastname@example.org .