I had a frame with an awful dent on the seatstay. Since the frame was potential scrap anyway, I had a go at fixing it with some DIY tube blocks.
Two blocks of wood, bolt together, drill a hole through the middle. Then place over the frame tube with plenty of grease, and slowly tighten the bolts whilst rotating the blocks around the tube.
It did work - there were some ripples in the middle of where the dent was (not dissimilar to the skin on custard... or your elbow).
It would take some fancy woodworking to make blocks for tapered tubes... and I don't think it would work at all for ovalised tubes since you can't rotate the blocks.