You can use mosfets as clippers, but they will start clipping at a higher voltage and will have a different clipping characteristics than diodes. (rest see above). if you don´t use the additional diodes the effect will be lost since the protection diodes start clipping long before the mosfet itself does.
So you can use mosfets "not in a normal way" that the mosfet itself does the clipping, instead the clipping is done by the (mosfet internal) protection diodes.
In the pcb there are soldered mosfets but they are not working as mosfets - like the normal way - cause the protection diodes override them. The "usual way" would be to connect a diode in series to avoid this.
For more info about the "history" look here:
http://www.diystompboxes.com/pedals/schematics.html
and scroll down to "Shaka Braddah III".
analogguru
There´s a sucker born every minute - and too many of them end up in the bootweak pedal biz.