This is so weird, I was just about to ask exactly the same question....
A friend of mine has taken my advice and moved from SmoothWall to E-Smith and used to have:
ifconfig -i eth2 hw ether 00e09872e436
in /etc/rc.d/rc.netaddress.up just before it sent out it's DHCP request but I'm damned if I can figure out where to put it on the E-Smith...
Any help on this would be greatly appreciated...