For the benefit of other people following this thread, I repost my test results here:
The fit is not perfect, but it is the best I could do using such a simple model:
I used the following constants:
KV: 800 (experimental)
Kt: 0.0119 N.m/Amp (derived from KV).
Req 0.3116 Ohm. (Experimental fit, actual resistance of one
(phase) coil measured on our dyno: 0.1633ohm)
b: 7.3795e-008 N.m.s (Could be neglected)
I0: 0.33A Small effect, mostly important for correct efficiency at no load.
Consequently, Kv and Req are really the minimum, but as you can see the fit is not perfect, which is why you need a dyno for precise measurement.