I ended up kind of making up a skill challenge on the fly that involved having a few seconds to judge your opponent (Insight, Perception) then the charge had two rolls: on in which the PCs could figure out a way to dodge the attack (Athletics, Endurance, Acrobatics) and a set Atheletics roll for the attack. I actually think it worked out well, based on my extremely limited understanding of jousting. All the PCs who were not very Athletic did well at observing their opponents and dodging, but couldn't actually unhorse their opponents. Since they're not trained in Athletics and presumably never jousted before, it kinda makes sense...