I'm in the same situation with a set of Shimano 600 wheels I got for $25. The rear has a flat-spotted Wolber GTX rim. I've seen the Park tool which expands off the hub's shell. Looks iffy. Thought I'd experiment with a plywood jig and C clamps to try to pull the flat spot out some. Curved pieces of wood to support contact points.
Nothing to loose, otherwise it's a new rim anyway.