It's difficult to know what is actually impossible and what is just difficult for most agents to do. In practice, most of us with IB+ tend to transfer to BAEC too book with partners, because BAEC redemptions are refundable, even though sometimes redemptions on IB+ can be cheaper. Therefore, there is a lack of data points from people who are both familiar with the programme and have access to better agents (by calling to the IB+ phone numbers for elites and by speaking Spanish).

I myself tend to use my Avios just on redemptions on IB metal or on domestic AA flights. For the latter I have almost always transferred to BAEC (I did manage to book a one-way on AA metal with IB+ once, though, but it showed up online and this was a couple of years ago). The IB+ award chart on AA is more favourable than BA's when there are stops, because you pay for total distance and not by segment, but in practice you mostly book redemptions for direct flights.