The easiest route in my mind is extra C-clamps. If anybody else is like me, they have a few sitting around at a time that they can spare. If you don't, they aren't expensive.
I have plenty of faith in empty C-clamps, but my advice is that if that's a route you're considering, don't take them down when you don't need them, just leave them up. If you put them up, crank them on, then you don't have to worry about them so much as if you're constantly changing them; then there's room for error if somebody were to not crank one down enough putting it back.
Where does the serious concern lie though? My experience with vertical booms is that if the
instrument isn't clamped on completely, the weight will
shift in such a manner that the
fixture doesn't free fall, rather it stays in place until you lift it, making it perpendicular with the pipe. Aside from that, if you just put the
safety cable around the pipe the light will still fall, but will only fall to the next light or bottom of the
boom. Even without that though, gravity will still prevail, and I'd guess that most people don't have audience members seated directly below vertical booms in the first place.
If the
boom isn't fixed, but temporary, my concern would be most placed in that if a light fell it would
throw the
boom out of balance when the
shock load kicks in and next thing you know you're
boom is toppling over.
Another factor is how the
boom is constructed. If it's a straight pipe floor to ceiling that's fine, but if it's vertical pipe mounted at the bottom with a stand off from the wall and doesn't connect to the floor, it certainly can't hurt to put a cable on fixtures there because they will be caught by the
bottom pipe that's perpendicular to the vertical pipe and the wall.