FWIW, in my (limited) experience, I have been successful in getting UA to endorse me to another non *A airline. (In this case, one of the KL AMS-LAX non-stops would have been the obvious option.) If you do all the leg work yourself (i.e., find the flight, reserve your seat, etc.), they have a hard time arguing with just printing to paper and signing the ticket over.

Of course, I never travel with checked bags, so have no idea whether you'd be able to get your luggage to the destination with any reliability in that case.