Can you weld? If yes you can add an idler pulley to slow the rotation. You'd still have to do the math as to what size pulleys you'll need but it could be done. I don't know if you'd be able to do it in one step because of the size of the pulleys.

Another option would be to find a suitable washer with a DC motor and refit into your dryer. Then you can worry about PWM. A treadmill motor would be perfect as the control is already done for you. Mounting may be a challenge but refer to first question.

If you were closer to Las Vegas, NV I could fix you up.