From the moment I saw the first trailer for Quentin Tarantino's Once Upon a Time... in Hollywood, I was the most excited about one moment. Not the promise of revisionist Charlie Manson history. Nor the pleasures of seeing mega-stars Leonardo DiCaprio and Brad Pitt hang out with each other. No, dear reader, I was the most excited about seeing DiCaprio, as has-been actor Rick Dalton, sing and dance on fake Laugh-In-esque variety show "Hullabaloo." I loved the set, the campy vibes, and the promise of DiCaprio going for broad comedy -- a mode he's very …