It's not like that at all - alternators turn freely, their output is related to RPM.  

Under cruising RPM, your engine won't "work harder" to generate the power needed to run the miner, that power is already being wasted in the voltage reg/charge systems.  Miner is drawing from the battery, alt is providing variable charging amps to the battery/+ circuit while running.  Simple, keep the charge rate > discharge rate.

Lights may dim because your 18" Subwoofing hasher is drawing high amps at stoplights, but it won't be noticeable at the pump.

I suppose if the miner is large enough - get a bigger alt, huge inverter, external charge controller, aux batteries, and a smaller pulley.

Picture this - ice road truckers running miners in their sleeper cabs for heat and extra BTC