Okay, since no one else has answered;
Based on what I've read about serial programming on a *nix based system, it doesn't seem likely that you can get a true, real-time, signal on the software side. I suspect that the best way would be to simulate the bit-stream using a pseudo-random sequence.
The Tx and Rx L.E.D.s found on embedded boards are physically connected to the lines they represent. However, there is no feedback of these lines to the OS as far as I know. (and if there is, it's probably implemented differently on different hardware.)
Here are some links to resources that may help you to determine when, in the program you're writing, you should start and stop the 'blinking' of the L.E.D.s
http://www.tldp.org/HOWTO/Serial-Programming-HOWTO/
http://en.wikibooks.org/wiki/Serial_...g/Serial_Linux
http://stackoverflow.com/questions/6...rial-port-in-c
