I know it's not a solution any arborist would recommend, because you want a new tree to sway in the wind, but I used the rope idea from my first post. I held the lower trunk in place with one rope and pulled the top of the tree the other way with another rope. The curved part of the trunk was just flexible enough to be forced straight. It has now been three months and the tree is mostly straight on its own. One more month should finish the job. I will then remove the ropes and let the tree go free. I'll keep an eye on it, though. On windy autumn days I could provide some loose supports so the tree doesn't whip around too much.
