Yes, this code does change the value of the PllCtrl1 register using mmio.  It basically reads and writes the hex values to or from  /dev/mem  It could be very easily made to work with the Voodoo4 and 5 as well.  Due to VSA-100 SLI incompatibilities though, it wouldn't be worthwhile of course.  The code could be edited to OC a Voodoo2 and overclock is SLI though perhaps, as Voodoo2 SLI does have driver support. ;)

I already know of this, it uses envirmonment variables however and not the IO register.  I was merely stating that the code you linked to could possibly be use to OC the Voodoo2 via the IO register. :)