Building a 20x20 matrix with vibrator motor coins
400 motors at 60mA per motor is a total of 24A maximum supply current. If you multiplex the motors then peak current will be much higher and could become impractical, so it will probably be best to drive them independently.   

To apply different PWM frequencies to each motor, every one will need its own switching circuit (MOSFET, flyback diode, bulk capacitor). It's only a small step from there to providing each motor with its own MCU, all of which can be networked together and controlled from a single point.

Depending on the physical arrangement of the motors, it might be better to combine several motor drivers on a single pcb, or just make 400 individual boards and arrange them however you wish. Each board could have an input connector and a pass-through connector, so they can easily be daisy chained in rows or columns without extra wiring. This is similar to how 'intelligent' LED light strips work.

400 individual circuits might sound like an expensive way to do it, but with bulk purchasing it should work out to only a few dollars per board (less if you get them manufactured in China!).