when we worked out a board recently it was really easy.
David, Stefan and I did it.
What we did:
Use superiotoo to dump the superio. Look at GPIOs. Notice that one, and only one, GPIO was even enabled. And it was enabled as output.
So we looked at the output value, inverted it, and flashrom worked fine.
This was a few minutes of debugging. It took longer to modify superiotool to know about this particular superio than it took to figure out how to write enable flash :-)
You'll just need to dump the states of the GPIOs on the SiS part, see which ones are enabled, see which are enabled as outputs, and for a first try, just invert them all ... you can work out which one is controlling it bit by bit.
ron