how about dyno pack dynos where the hub is directly bolted to the dyno pack? no wheels on the car.

Yes this is how they measure them in Sweden at rototest.

https://www.rri.se/

For instance, check out the power train loss on a BMW M5.
Power train loss 7-8%.

Wheel power 472PS
Stated engine power 507PS.

At this power level with weight 1.85T & manual gearbox, 35bhp power loss is reasonable!

Jaguar XJ40 4.0L with Automatic gearbox.
Power train loss 18%.
Weight 1.75T
Wheel power 205 PS
Stated engine power 249PS (possibly only 235.....)
So again probably 30bhp power train loss, but it's AUTOMATIC.