dvsource-firewire supports the concept of a tally light by way of a message printed to the consoile. With a small wrapper script you can capture this output and take some action.
Wouter Verhelst suggests an LED on a serial port as a light. Wouter writes:
wire a LED between pins 4 and 5 on a 9-pin serial port, then if you open the port, the LED will go on (provided you did it properly). Parts for that will cost you about €10-€15 a piece (USB serial convertor included), which should not be prohibitive and certainly is much easier to use (laptop screens tend to blank, or produce a huge blare which may be disturbing to the audience).
With that, all you need is doc/examples/tally-light.sh in the dvswitch git repo.