Here's a detail of the top rear side of the seatpost. On close inspection, it seems as if a nut was welded there originally, so I will probably try to drill out the bolt stump, tap the nut, and put in a new bolt. Perhaps someone can tell me how long that bolt sticks out originally?
Also, does anyone have an idea as to how to remove the chainguard? I have a very bad feeling it's secured by the fixed cup. That's going to be fun...