Any keyboard will technically work once it boots into the OS. The issue of having the original keyboard on the older iMacs is that the newer aluminum keyboards would not initialize prior to the boot prompt. So you weren't able to do things like PRAM reset or boot to a different disk without the old keyboard. Not sure if thats what you're referring to but I had never found a workaround for this.
If its not working once you boot into an OS, you either have a hosed keyboard, hosed usb port, or hosed OS (which used to happen a lot with people hacking usb kext's in order to jailbreak phones).