Actually my idea was in terms of scale, if anything, it would need to be scaled UP, not down. The particular Sprag bearing I am refering to links to a 1/4" shaft, transmits about 5-10 lb/ft of torque easily, and has a normal differential speed of 35,000 RPM. These would not need to transmit the full load of turbo spooling, only support a median-level turbine floor speed.
Think RC Car.
Now think harder.