You mentioned the great Frank Zappa/Alice Cooper onstage gross-out contest in the late 60s or early 70s. Here's the version I was told back in the 60s/ early 70s
Seems that Zappa had Alice come out on stage at some concert. But instead of playing a song together, they decided to have a gross-out contest. I don't recall the preliminary feats of disgust, but eventually Alice, in what he assumed would be the winning play, took a dump on the stage. But not to be outdone, Zappa got a spoon (most bands usually have silverware onstage during a gig, right?) and took a big steamin' bite. Frank won the contest.
Back in high school, around the time this supposedly happened, I heard that story repeated several times by friends and school mates.
But in the 1980s, when original Mothers drummer Jimmy Carl Black moved to New Mexico, I asked him about it. About halfway through my question, Jim took over and told the rest of the story. "SO it's actually TRUE?" I asked incredulously. "Fuck No!" he said.
Later in his band of fellow Zappa vets called The Grandmothers, Jimmy and Don Preston recorded a song called "The Eternal Question" (originally titled "What Was Zappa Really Like?") -- which contains a refrain, "And did he really shit on the stage?"