Long story short, I never got the booking tool to work with an x-AMM ticket. I'm glad other people have been able to, but I wasn't.

AA confirmed that it was a valid itinerary. I ended up doing it all with them again over the phone, then pricing it in Canadian dollars to ensure it wasn't priced at the higher x-US rate, then spending another couple of hours figuring out how to get it issued.

But it worked in the end, so that's what matters.