Ok that may work.
Copy the ATIROMFlasher_SL.kext file to yor desctop.
Then right click or two finger click or what ever you use and select edit package.
Then see if you can find the com.apple.boot.plist file i mentioned above and change the macmodel ID
Then save and copy back.
If you run in to permission errors then the file will need changing first with a sudo command which i can instruct you to do.
(as i havent opend this kext file before i have no idea if the identifier scrip is even in there but its worth a try)
If the com.apple.boot.plist is not in there the macmodel ID could also be in a diffrent plist file.
Maybe info.plst, im just guessing as i havent done this before.
Good luck.
If all this fails we will have to try to spoth the Imacs model ID instead of chaning the installer ID.