If you have your challenge, it's valid the entire game, is it not? If Williams hadn't used it on the safe/out at second early on, it would have been available for him to demand a challenge on the pickoff. I believe once it's been used, you can ask nicely but the umps don't have to oblige. Do you think the umps were like "yeah we could grant you this critical review, but the Cubs guys already went through their handshake line and its after midnight so nah"?