Yes it is possible - I have made and tested this RS232 interface yet (see the bootup messages from PMON below).
Please note you will loose the warranty by this modification as there is a warranty label that you will have to remove.
There's no impact to the router's mainboard as the expansion card is connected through onboard connector only and it can be removed anytime.

You will need just few electronic parts to make the expansion board.
- UART or DUART chip (16c552,16c550,16c450,82c50,...)
- XTAL 13 MHz (or close to 13 MHz)
- MAX232 or equivalent (two pieces if you plan to use DUART)
- some passive elements - capacitors, resistors, connectors

The schematic will follow later on (as soon as I convert it to electronic form somehow...)


Messages from PMON during the boot:
PMON version 5.3.22 [EL], LSI LOGIC Corp. and Broadcom Corp.
Compiled on Fri Apr 25 15:24:08 2003
CPU type 4710.CPU clock frequency 125 MHz.Avail RAM 16384 KBytes.
NVRAM: AMD 29lv320DT 2Mx16 TopB.
Visit www.carmel.com for updates.

~Downloading os image in 3 seconds
Using specified MAC address.
et0: Broadcom BCM47xx 10/100 Mbps Ethernet Controller 3.11.19.0
MAC Address: xx:xx:xx:xx:xx:xx
Opened ethernet
Downloading from ethernet, ^C to abort
Downloading image time out
Boot os from the flash
CRC OK
Uncompressing..................................... ................done
Doing command call 80001000