I'm sure people have mentioned this already but I feel people aren't recognizing just how easy it's going to be to get tickets Friday night if you hang out by an exit after Caifanes's set finishes with $100 cash on you. Obviously plenty of Caifanes fans are going to stick around the weekend, but given the dynamics (one of the biggest Mexican rock bands ever, reunion after 15-year absence, only U.S. date scheduled, people being forced to buy 3-day passes) there are going to be hundreds if not thousands of people heading for home Friday night that would be happy to part ways with their wristband for some extra cash.
Anecdotally it sounds like it was doable to find a Jay-Z fan willing to resell last year. Back in '04 this was how I got a Sunday ticket for $20 from a departing Radiohead fan (although back then you could buy single-day tickets, so never was sure why that person bought and changed their mind).
Getting the wristbands off and then back on is going to be the biggest challenge, and the main reason why if I didn't have tickets I'd err on the side of caution and get tickets ahead of time (well that and I'm not missing Friday). BUT, if you're an unlucky SoCaler that didn't get tickets and don't wanna pay the scalper mark-up, I'd recommend asking your friends to be on the lookout at the end of Friday and see if there's any skinny Caifanes fans looking like they're on their way home.