Heck, it's even better than that. You can install Tiger on an original 233MHz iMac from 1998 and it'll run just fine, thank you. Well, sure, it's a bit sluggish, but it all works. The trick is just to load the OS on an appropriate HDD while it's in a supported Mac, then physically install the HDD into the iMac. No need for monkeying around with XPostFacto (though that is a pretty useful tool if you need OS X on Old World Macs).
As soon as I get a copy of Leopard, I'm going to see if this method still works for unsupported systems like the tray-loading iMacs and B&W G3s. Betcha a nickel it will.