|so is there a way to install from an external firewire hard drive with the open firmware change?
The whole idea of this Open Firmware-based technique is that you don't need to modify the original installation DVD because your system will pass the check of a CPU frequency not less than 867 MHz.
A Disk Image can't be booted from, but it can be restored to an empty partition using Disk Utility. Such a partition will be bootable (using the steps described in the previous article). I've done this for a few installation tests before upgrading my work system.