It turns out BA Executive Club has a separate chart for multi-airline awards, and oh my do they sting you.
The chart can be found here:
This is economy, regardless of one-way or return. Premium economy is 1.5x, business 2x and first 3x.
I came across this when searching for New York – Boston roundtrip, expecting 9,000 Avios. I selected US on the outbound and AA on the inbound and got 30,000.
The solution in such cases is to book separate one-ways at 4,500 each. More difficult are those instances forcing multiple partners on a one-way. The BA website does not usually mix partners in its one-way searches though it should be possible to book online or on the phone. Beware!
See more on this at Head for Points, where he posits there might be some rare cases where this chart can be useful.