I am steaming mad right now, and should probably calm down before posting, but I will try to moderate my language and tone.
After my 4th call, and a total of more than 3 hours on hold, I got an answer: this temporary name change waiver for flight credits can only be used for AA mainline services. So my non-stops between ORD and DSM on Eagle are not eligible, and a connection in Charlotte would be required. This was not mentioned in the email.
Also on my call 2 days ago about this, I was told that my hold was being extended to allow more time to complete the booking. Great - that took the pressure off. But when calling again today it transpired that the "hold" was just for the flight times, not the fare, so it was repriced as a fresh booking today. Which stretches the definition of "hold" in my view, and makes it worthless.
All in all a terrible customer experience, and a waste of several hours of my time. Guess I'll hold out some faint hope of the formal conversion to trip credit, but that will be too late for this itinerary. I'll be booking my daughter on UA instead.
One last point: for anyone trying to do this who encounters an agent unaware of the promotion (and I've had several who strugged to find it), the magic phrase to use is "name change assignability promotion".
Pissed-Off in Chicago (I almost made it...)