Might be worth a try to adapt a replaceable (bolt-on) hanger to the broken dropout so that any future damage can also be easily fixed. If appearance is not a big concern, you could also use a steel "claw" derailler hangar from a older cheap bike that fits over the axel, sandwiched between quick release nut and the frame.

If there is no bolt-on solution, brazing-on a replacement wont really work for aluminim but you would likely be OK to have a replacement hangar TIG welded on without going through subsequent heat treatment of the entire frame that is needed for tubing or full dropout repair of aluminum bikes.