It's pretty easy to attach a serial console to the router. There are two serial ports on the board, ttyS0 and ttyS1. The console is going to ttyS1 which conveniently also has the header soldered on.
To communicate with the router use the following settings in minicom: 115200 8N1 , software flow control.
Yes, I know my interface is dodgy and I could have made it smaller and neater, but I was in a hurry and built it with what I had lying around ;P
The serial console will display the boot messages and end with a logon prompt. You can login with "root" and "password". The newer versions of the routers automatically run a program called lrcfg which is a simple configuration tool.
Next: Access the CFE