Thanks. I'll try drilling holes but the crack is in the dent and extends out of it pretty long already. I was also thinking about jamming a wood dowel or something down into the tube. But just looking at the frame I'm also wondering how important the seat tube is structurally. It's hard to imagine the frame collapsing, and with the size of the downtube I wonder if the bottom bracket might be supported from pedaling stress well enough by it alone. I don't know what the normal stresses are on the seat tube, or how a lack of one might affect things.
It's just so much lighter than my steel frame that I'm transfering it's parts to, so it's hard to throw away and a tempting experiment from the trash pile that I can't justify spending money on.