To anyone finding this from Google - from my experience, any flight linked from ITA Powertools to AA.com that comes up as unbookable is also unbookable via bookaa.amadeus.com, although Amadeus will actually try to ticket it.
I had another experience with issues with booking through ITA Powertools, and the first result of my Google-Fu resulted in my own post, of course! This time, I was actually able to get a PNR out of bookaa.amadeus.com, but it did not ticket (plus aa.com wouldn't even load the itinerary, I believe due to a 5 minute discrepancy with the flight time on a BA codeshare flight). My usual trick of converting a codeshare to the prime flight number worked to get the result on AA.com, but the fare was a bucket higher, so it was going to charge me about $60 more. Being that BookAA put a pending charge on my card (in EUR) and I got a PNR this time, I gave it a few hours and then called into AA - they said they did not charge my card and requested payment again, this time in the correct USD amount that ITA Matrix had provided, and had it ticketed. Overall, it was a bit of a faff, but easier than feeding in the exact flight numbers I pieced together.
Now to just hope that the first payment doesn't actually post, which is never guaranteed with AA.