The Win8 installer does not list any specific missing driver, it is just (what I think is) the generic "your system needs a driver that is missing" dialog; sorry, I didn't write down the specific message.
I also tried pointing to the two folders mentioned above, as well as a couple of their sub-directories (from the Vista installation), but quickly received the error that no additional drivers were found.
My plan at this point is to use an SSD for a new Win8 boot drive, then modify the installation to move the user files to the Raid array; this is pretty well documented in this link:
Thanks for the reply, if I have any updates I'll add to this thread - but in the end, I could not seem to find a way to get the Win8 installer to use a new or pre-existing partition on the Raid5 array controlled by the ICH10R chipset.
Sorry, never mind - the issue was not with the Intel RAID drivers.
After going through the process of installing a SSD for Win8, expecting that this would be a work-around to the missing driver message, I still could not proceed with the installation. At that point I didn't know what driver was missing, but tried pointing to a few folders on my Win Vista drive, and sure enough what was missing was the SATA/Raid driver for the on-board Gigabyte chipset. Once this was loaded I was able to proceed as expected.
I guess the clue all along should have been the fact that I could already access the Intel RAID array in the installer tool when browsing for drivers.