This would be perfectly fine for normal users who have just one such router in her LAN, but it didn't work for me since MAC addresses have to be unique in a LAN.
The nvram set command can be used to change the MAC address, but it only affects the MAC address displayed in the DD-WRT control panel, it doesn't actually change the MAC address. After some fiddling, I found that I need to use the ifconfig command, so the full command is:
ifconfig br0 down
ifconfig br0 hw ether 00:25:9C:CA:23:0D
ifconfig br0 up
The LAN MAC should be the same as the Wireless MAC, so that's what I used. The commands can be save as a startup command using the DD-WRT interface. And here's a screen capture of the new setting in effect.
Since I have more than 10 WRT160NL's, I had to go to each one to change the MAC address. This is a bug in DD-WRT, but since it only affects people with multiple routers, I think it may be a long time before it gets fixed, so my workaround will do for now.