A tough nut to crack in a precision way. There is a tool designed to hold the inside diameter of a tube on a lathe or, drill press. It's called an expanding chuck. I have a couple but both larger than the ID of a seat post, and, they're designed to hold one end of the tube while the other end is supported on a tailstock center. Not an option on a seat post. An expanding, rubber sanding drum probably won't work as most of them are designed to be tightened with a nut on the outboard side of the drum, the part that would be buried inside the seat post. You might be able to make something but the trouble might be more than it's worth. Held in a metal lathe, the portion held in the chuck is the same portion you need to turn down, making that method difficult. Even with all the tools and machines I have available, I believe I'd revert to the hand-held sandpaper method if I were attempting this. A seat post is not as critical to keep perfectly cylindrical as a stem would be, of course.

Edit: I spoke too soon. just went down and looked at my expanding sanding drums and two of them indeed tighten with a nut on the inboard, or shank side of the drum, making this a viable option if you can find one of the right diameter. Held in a drill press, the other end of the post, the clamp casting area, would still need to be supported and I think I would use Otis' split block method, clamped to the drill press table and perhaps lined with some felt to avoid scratching the post up near its top. This might work and would give you a level of precision greater than hand-held sandpaper.