I've tried this and the results are very disappointing. Some small motors just won't turn at a few rpm because of internal friction, unless they have precision shafts and ball bearings.
Most small to medium DC motors run several thousand rpm at their rated voltage, and rpms are proportional to average motor voltage (no load). This means that you have to reduce the average voltage by over a factor of roughly a thousand to get to a few rpms. You won't have any usable torque left at that low rotational velocity.
On the other hand, stepper motors are nearly silent and can rotate as slowly as you like.