Add static IP settings to custom rom
Save the following in one of the init scripts in the ROM

ifconfig eth0 192.168.1.100 netmask 255.255.255.0
route add default gw 192.168.1.1 dev eth0
setprop net.dns1 192.168.1.1
ip addr show
ip route show


Where eth0 in this case, is an ethernet adapter attached via USB.