Two further updates:
1. It's not possible to edit the file correctly with "vi". One must use the "echo" procedure described earlier.
2. "One-armed" routing also works on Zaurus C700 with the Sharp ROM (i.e., assigning multiple IP addresses to a single interface and routing between them). One simply needs to enable IP aliasing (not NAT) with "ifconfig [current LAN interface]:0 [new IP address] netmask [new IP mask] up". For instance, if the Ethernet interface is "eth0" and the desired, new IP address is 10.2.2.2/255.0.0.0, then one would enter "ifconfig eth0:0 10.2.2.2 netmask 255.0.0.0 up". One can add additional IP addresses with "ifconfig eth0:1 ...", "ifconfig eth0:2...", etc.