The inertial system is easy to calibrate, its just like driving the car on the road with the GPS dyno. You have accelerated a mass of Xkg to speed Ykph in Zsec, and you measure the resistance of the roller bearings with a string and pulley system. But that is all it does, one pull though the revs then slow it down for the next one.
The brake dyno can hold the motor at any rpm, but you need either those electrical retarders or the water (or oil) ones, all off heavy vehicles. Mercedes offer water ones on their trucks here, the heated water goes into the radiator and around again. Telma make the electrical ones, but they're all worth a lot of money. I'll look at using a torque converter out of an auto box, its the same basic system. That will measure torque with a load cell OK, but you need to find out how much is lost in the drive.
Maybe we can set this up on a roller for a chassis dyno-
As for insurance on car work... Hahaha, these guys own KE70s, you don't expect us to get PAID do you??