My LBS has had a few Al bikes with snapped derailleur hangers. Al is just not strong enough for the job, which is why better ones have replaceable hangers.
To save the hanger from over-stressing, use a breakaway bolt, which will snap before the hanger.
If this happens far from home, it is the kind of situation where you would rejig the bike as a singlespeed, thanking yourself that you opted for horizontal dropouts to tension the chain.