Oh interesting and timely thread.
I have a Schwinn Prelude with a slight top tube dent I was considering trying this method on.
Did you rotate the frame around the blocks in the vise?
As for filler, I remembered car assembly lines used solder and flowed it into seams between welded panels and then made smooth with large soldering irons and files.
I once asked about this method to fill gouges to a ball burnished AL frame chainstay where Bondo wouldn't suffice. I got no feedback if it would be applicable to frames. Heat might be the issue, needing enough to flow the solder.
Metal filler has definitely been used to fill in small dents and gouges on steel frames. Pics from a LBS website shows the end result. No mention of the metal though, I wonder if it's solder?