After months of testing and pondering, here's how to change your ethernet hardware (MAC) address on any computer Apple's LAN Utility doesn't work on (basically anything after the xx00 series, and this probably works on computers that the LAN util  does work on, too :)).

Open up ResEdit

Open your System file

Add a resource, with the name "eadr"

Create a new resource in that resource (I forget if resource is the term or not, but eh, you get it ;))

The Resource ID should be that of your ethernet card's eth interface, in other words, if you have one card, or an iMac or something like that, the ID is most likely 0 (zero - eth0), so try that first. If you have more than one card, or it's not 0, you probably know what it is.

In the actual hex editing window, type the ethernet address you want in the hex part of ResEdit, it should give you some whackey characters to the right in the ASCII portion, that's normal :)

Save the file, and quit.

Restart, or reset the .ENET driver via MacsBug (I have no idea how to do that, does anyone? I'd love to know for testing perposes)

How it works:

When the Mac opens the .ENET driver, the driver looks for an 'eadr' resource in the System file with the resource ID that matches the slot number of the card (eth interface number, usually 0 [eth0]). If it finds one, the driver substitutes the number in this resource for the Ethernet hardware address and uses it until the driver is closed or reset (via MacsBug or a restart).

It's so nice to finally figure this out, He's been at it for months and months!

Enjoy, and use sparingly (Conflicting ethernet hardware addresses on the same LAN = not so good idea)