If you goto Rovios support page and click manuals, there are 2, the second one might be helpful for you,
Does the hardware on the Rovio have any possibility of connecting other devices electrically? I imagine that the existing light on/off ability can be taken over to do something else, but I am talking about unused available connections. Then the next question would be if the software integrates with the addtiional port.
I imagine the ideal situation would be an actual data port (perhaps a TTL RS232) that provides 2 way communication with the host computer.
There are at least two serial ports (UARTs)
/dev/ser0 is used as the "NS" com port which I assume is the North Star system.
/dev/ser1 appears to handle the "mcu" commands for motor control. Similar to the RSMedia.
Once someone takes it apart it will be interesting to see exact chips.
"mcu" (no .cgi) appears to be the glue for lower level robot commands. It requires a long string of numbers in a "parameters" arg. I think it is sent directly to the serial port to control the robot motion.
This needs more exploration (used often in rovio.js)
So, yes, there appear to be 2 serial ports and you MAY be able to add one way communication without needing to alter the firmware.
Since the firmware appears to contain portions of open source code (see this thread), they should be releasing at least partial source code. I suggest dropping them a line and asking for it.