Unfortunately, pirates are a little more sophisticated than you’re assuming. If it was just some guy in the audience with a camcorder, you’d be correct about the markings not being very effective. However, some of the popular and higher quality piracy schemes require cooperation from the projectionist:
Telesync - Wikipedia – Movie pirate sets up a tripod-mounted camera in the projection booth and directly hooks into the sound system.
Telecine (copying) - Wikipedia – Movie pirate copies the movie reel directly.
He never mentioned any such trick (and I think he would have told me). He did this for a chain of theatres in the area, and I know he mentioned that on certain days of the week, he was sometimes hard pressed to get all the films done in time for the afternoon matinee showings.
I’ve noticed big sound drop-outs (several seconds long) at a particular theater we go to, but that might just be a poorly trained projectionist at work. The Indy movie had probably half a dozen instances of it.
It would be relatively easy to do it, especially if you were a single-screen theater and the projectionist was only responsible for one movie at a time. Most theaters didn’t show both mainstream movies and porn, so he would have to obtain the naughty frames somewhere else, maybe from someone in the projectionists’ union. However, if he got caught he could have gotten in a lot of trouble back in the days when this was more common, depending on the region.
It’s FAR more common to find old prints with frames of gore or T&A clipped out for some projectionist’s personal collection. Sometimes just a few frames are missing, and sometimes it’s whole scenes. Hard to say whether they were snagged by collectors or self-appointed censors in some cases.