I've gotten the vt82c686 working, but at the moment HP still has my laptop, and I don't have a laptop->ide hard drive adapter. Anyways, you'd need to modify all of the pci ids in the 8231 code to match the 686. I also had to remove the usb and nic support to get the code to compile, and there may have been a couple other little changes, I can't remember atm. As far as the northbridge goes, if you're lucky, it's just the same procedure, modify the device ids from the 8601 code.
-Corey