Just to give another data point.
Award ticket for 1 adult in business booked on ZH and OZ. I called multiple times to both OZ and Avianca. The method that ended up working was to call the US based OZ agent and book an infant fare. They tried to get me off the phone multiple times by saying it was issued by TACA, and then that it was actually issues by ZH since they were the first carrier. Just keep telling them that it's possible and don't take no for an answer. Another excuse was mis-matching fare classes...after about the 3rd time of being put on hold the agent was able to successfully get the reservation. Specific to OZ they then give you an email address and reservation number to email them your CC and DL to purchase the fare. Wasn't too painful once you get an agent who knows what they are doing.