Well, I've never actually bought or built one, but I remember seeing
something like this being published in a hobby electronics magazine as
a hobby project years ago....I think it tried to exploit the fact that
some average level of the video or audio signal was usually different
during commercials. I think it may have been designed to mute the
audio during commercials...perhaps it detected a different DC black
level on the video or a change in the amplitude and/or shape of sync
pulses. It's been a long time and I don't even remember the name of
the magazine, but hopefully this is food for thougtht.