I'm just now ready to take a look at it...yep, that's the "whitelist" I'm talking about.
I'm *thinking* that the very last entry matches the firmware version string that it actually installs, but I'm not sure. Is your currently-installed firmware version 3.0.f1? I don't know if there's an easy way to find it, but I think if you run CopyROMs it will dump that firmware into a 4 MB file (four repetitions of the 1 MB firmware) and the version will be a string in it somewhere. It might actually be a good idea to dump your current firmware anyway, just so in case something goes bad we might be able to reflash your chip with the entire contents of what used to be on it (I have a Willem programmer, we could probably figure it out).
Since the original firmware update 1.0 came out in December 18, 1998, and all the rest of the firmwares after 3.0.f3 in the 1.2 list are dated after that date, I'm pretty sure the very last entry in the 1.0 whitelist is the version that it actually installs (and same with the 1.2 whitelist) -- so you probably have version 3.0.f1 currently installed, since that's the latest version in the 1.2 list.
What I would do is replace the very first entry in the 1.0 whitelist ("Apple iMac Open Firmware 3.0.f8 built on 08/06/98 at 12:04:16") with the very last entry in the 1.2 whitelist ("Apple iMac Open Firmware 3.0.f1 built on 04/01/99 at 16:47:10"). Be sure to leave the NULL byte in place that's directly after it, and don't change the size of the data fork at all. Just replace existing characters.
I can whip it up and send you the file in a sec...this is getting exciting! (P.S. it's weird that 3.0.f1 is actually newer than 3.0.f10...I wonder if they meant to call it 3.0.f11...but it's a good thing they did it that way because it would have lengthened the string and made it more difficult to patch)
Edit: here's the link to the patched updater program:
https://www.dropbox.com/s/3ncm464351gpetk/iMac%20Firmware%201.0%20Downgrade.cpt
Extract it (it's a Compact Pro archive) and it'll give you a new iMac Firmware Updater program. Copy the contents of the 1.0 disk image to a folder on your hard drive, but replace the updater application with the one I just bundled, and run it. It should allow it to install the 1.0 firmware over the 1.2 firmware...
I'm *thinking* that the very last entry matches the firmware version string that it actually installs, but I'm not sure. Is your currently-installed firmware version 3.0.f1? I don't know if there's an easy way to find it, but I think if you run CopyROMs it will dump that firmware into a 4 MB file (four repetitions of the 1 MB firmware) and the version will be a string in it somewhere. It might actually be a good idea to dump your current firmware anyway, just so in case something goes bad we might be able to reflash your chip with the entire contents of what used to be on it (I have a Willem programmer, we could probably figure it out).
Since the original firmware update 1.0 came out in December 18, 1998, and all the rest of the firmwares after 3.0.f3 in the 1.2 list are dated after that date, I'm pretty sure the very last entry in the 1.0 whitelist is the version that it actually installs (and same with the 1.2 whitelist) -- so you probably have version 3.0.f1 currently installed, since that's the latest version in the 1.2 list.
What I would do is replace the very first entry in the 1.0 whitelist ("Apple iMac Open Firmware 3.0.f8 built on 08/06/98 at 12:04:16") with the very last entry in the 1.2 whitelist ("Apple iMac Open Firmware 3.0.f1 built on 04/01/99 at 16:47:10"). Be sure to leave the NULL byte in place that's directly after it, and don't change the size of the data fork at all. Just replace existing characters.
I can whip it up and send you the file in a sec...this is getting exciting! (P.S. it's weird that 3.0.f1 is actually newer than 3.0.f10...I wonder if they meant to call it 3.0.f11...but it's a good thing they did it that way because it would have lengthened the string and made it more difficult to patch)
Edit: here's the link to the patched updater program:
https://www.dropbox.com/s/3ncm464351gpetk/iMac%20Firmware%201.0%20Downgrade.cpt
Extract it (it's a Compact Pro archive) and it'll give you a new iMac Firmware Updater program. Copy the contents of the 1.0 disk image to a folder on your hard drive, but replace the updater application with the one I just bundled, and run it. It should allow it to install the 1.0 firmware over the 1.2 firmware...