Thanks for posting the link to the right post. I do remember reading this, but I had missed the attachment at the time.
I was able to modify the V8 ISO to include this new Mac OS ROM in its System Folder. I did that by mounting the disk image with -readwrite under Leopard, which still had HFS write support. However, when I try to do the same to the embedded Macintosh HD image, I get: hdiutil: attach failed – Operation not permitted.
I can mount the "Macintosh HD" img embedded on the ISO as read-only, but not as read/write. I will see if I can manipulate it better on one of my classic Macs, but any pointers on how to slip the new ROM file in there from Leopard would be appreciated. And yes, I did try running hdiutil as root.