The PWM was suggested fully aware of the requirement.

In simple terms the supply is 12V and the motor is 6V.

So a PWM with a 50% duty cycle will produce exactly what you want. The PWM circuitry itself can be powered by the 12V and the motor in its out put will see an effective 6V only - the windings will do the averaging of the PWM pulses and only thing you have to experiment is to find the optimum PWM frequency that will deliver what you want with the least noise - if thats an issue.

I do believe that this solution is workable.

Dropping 6V with diodes - I dont think you plan to use 10 diodes ! Maybe a sutably rated zener ? OK power rating of the zener will have to be seen if this is workable.

Raghu