Software: a nifty trick in this Slashdot comment:
… This reminds me of an old trick we developed to use on the Amiga on a public-access cable channel. The software was under development and crashed occasionally, so rather than having a flashing guru meditation up on a local TV channel until it was rebooted the next day, we came up with a plan, that would probably work on a Windows machine as well (or just about any other system)
The idea was that while the software application was running, it drove a continuous 1khz tone out the audio port that kept a relay energized (that kept the signal on-air). When the system crashed, the audio output stopped, which meant the relay was no longer energized = video signal switched back to a stock SMPTE bars signal from a test generator.