This is very frustrating I am not sure if others have better solutions. A letter from a lawyer to corporate might elicit a response but seems extreme (but then so is AC's intransigence to date).

Seems we need a well-crafted question that prompts the correct response (fix the system). Might be something along the lines of (address to Customer Solutions, CEO, COO and AMEX customer service as well):

I have been refused acceptance of VXXXXLT fare by AC for the Worldwide Companion Pass both online and by several agents including companion pass fare agents. Attached are the terms and conditions for the WCP as published by AC on XXX and AMEX on XXX, please point me to the specific term/condition that indicates that this Latitude VXXXLT fare does not qualify for UCP use.

Anyway--good to know the details of this and perhaps several queries will prompt a resolution. I hate it when they can't give you a reason other than "it won't work" or "it doesn't qualify". . .

Good luck.