Using WL-500gP V2 internal serial port for Arduino?

Hello,

I would like to know if the Asus WL-500gP V2's serial port is locked (for the bootloader/kernel), or if it can be used like a normal ttl serial port? I think the serial port can be used like a terminal if telnet/ssh doesn't work, but can it also be used for example to connect a microcontroller to it?
Or would I have to switch off the terminal output? I think I wouldn't like to do that if something sometimes fails and I need to get in by serial. I don't mind if the serial port outputs any text at boot, I would just like to read temperature data through it.
I also probably need a level shifter if I want to connect a 5 volt microcontroller as I think the WL500gPV2 serial port is 3.3v TTL.

And no, I can't really use a usb->rs232 converter, I use a 3G dongle as my WAN connection and it connects as a ttyUSB0-2 device and if I have the serial converter attached it will disturb the 3G modem and it won't connect until I remove the usb-serial converter.


So my question is, is the serial port (the pins on the circuit board) locked, or can they be used to read output from another device? I'm actually using openwrt at the moment, but they should handle the serial port in the same way...