Good luck! I'll be excited to hear how it goes. Sorry about not being able to help you until now, it's just that I work during the day 
You'll probably have to change the date string to match too. I think it compares against the entire string, so hopefully CopyROMs will let you get the string out of it. Alternatively, I can decode the update file from 1.2 and see what version it actually provides. I might do that...
Edit: Actually, if you open up the "iMac Firmware 3.0" file with a text editor, you can kind of see what firmware it installs, I think:
1.2 says "BuildVersion=0x000013F2" -- matches up with 3.0f2?
1.0 says "BuildVersion=0x000012F3" -- matches up with 3.0f3?
I'm thinking maybe the 1.0 update included its own firmware version in the whitelist, but the 1.2 update didn't...I'd be curious if an iMac with 1.0 firmware allows you to rerun the 1.0 updater over and over again. I bet it does.
You'll probably have to change the date string to match too. I think it compares against the entire string, so hopefully CopyROMs will let you get the string out of it. Alternatively, I can decode the update file from 1.2 and see what version it actually provides. I might do that...
Edit: Actually, if you open up the "iMac Firmware 3.0" file with a text editor, you can kind of see what firmware it installs, I think:
1.2 says "BuildVersion=0x000013F2" -- matches up with 3.0f2?
1.0 says "BuildVersion=0x000012F3" -- matches up with 3.0f3?
I'm thinking maybe the 1.0 update included its own firmware version in the whitelist, but the 1.2 update didn't...I'd be curious if an iMac with 1.0 firmware allows you to rerun the 1.0 updater over and over again. I bet it does.