Woo-hoo! I think Gusto and I had a little bit of a breakthrough by changing up how I was asking for the behavior. I had been clicking him in a sit pretty, putting his front feet on the bar. The issue I was having was that he was either leaning on it (because it was there, not because he needs the support normally) or flat out throwing his feet at it. There was no way I could have held it off the ground - I needed to press it into the ground or it would swing away. I was waiting for the feet to wrap around a bit and clicking that.
Instead, what I started doing was asking for the sit pretty directly behind the pole, and luring him with the treat to lean around the pole for his reward. He's not leaning on the pole at at really, but what is happening is that often he winds up with the pole tucked in between his elbow and chest, and his entire foreleg wrapped around it a bit. Not tight enough to hold, but far more what I'm looking for.
I'm giving him a break, but can't wait to see if this keeps moving in the right direction later! It's the first time since he started using his front feet to target the pole that I feel like we've taken a step forward with this trick!