So after thinking about the other day, I got on Simon to school him at the house, looked at the round pen, back to the riding area, then just rode him into the round pen. I left the gate open thinking we wouldn't be in there long as the last time I'd lunged him in there he'd been a hot mess. Instead he was pretty chill and we had a good session.
Basically, I figured out some things about his canter. My intent was for relaxation but as soon as I dropped the reins and asked for a trot, Simon thought about taking off but then decided it was easier to just pack me around calmly. We kept this calm and worked on steering off my seat and some other things, just with loose reins. He did all that super awesome so I started asking for canter transitions. We were able to get about 20 in and I just let him figure out his balance and things worked out fine. At least I can get SOMETHING done and I learned a lot about what he likes, knows, prefers, etc. It was also a hellacious leg workout so I can see us doing this again.
Maybe we can actually DO our cantering at the trial... maybe.