Only fix I can think of would be a piece of steel plate shaped and then machined away to fit into the arc around the axle cutout. The machined indent would allow the lower part of the plate to sit below the broken section. This could then be drilled and tapped for the derailleur. The dropout could be drilled and tapped for a small bolt just above the broken part to help stabilize the repair and hold it in place when the wheel is removed.