Hydroforming with a Pressure Washer
 

Ok, I bumped into this video on YouTube a couple of days ago. And it got the mind spinning... I know, DANGEROUS!!!


Obviously their welds leaked a little bit at high pressure. But that probably doesn't matter. STAND BACK when working under pressure!!!

BikeForums? Well... I'm not sure that uncontrolled pressure forming would be that useful in frame shaping, and fabricating molds for home use would be a lot. However, cyclists have issues with dealing with random dents here and there.

If one is dealing with the main frame, then making plugs would be helpful. I don't know the pressure needed to take out a dent, but probably not extreme. One could make thread-in bottom bracket cups to seal, or for attachment fittings.

Many seatstays or chainstays have vent holes that could have a tube brazed onto for pressure. Or, perhaps a rubber sealed clamp-on fitting.

Of course, getting the water out of a tiny vent hole would be a pain and would probably require an oven.

You could plug end of frame tube that is difficult to access by freezing it in a block of ice to seal the tube end. You could even do this in combination with placing a pressure connection hose that passes through the ice plug, into the tube so that the pressure washer can be externally connected to the tube to pressurize within.
I've also had some limited success with removing dents by plugging tube end with ice, adding more water to completely fill the tube through a downtube water bottle boss so the new liquid water is in contact with the dent. Seal the water inside the frame with screws through the boss and then let it freeze. Expansion of water creates high pressure within as it freezes, just be careful not to split the tube. Helps if you live somewhere that gets down below 10°F outside in winter or if you have a large chest freezer.

We used to do dent removal in Motocross exhaust pipes by plugging the ends and pressurizing them with air. Then you slowly heat the dent area with a torch and watch the dent come out. You had to be careful not to overheat the metal to the melting point or it would just blow out a hole.