I feel like I should know this, but:

I'm looking at an ARN-LON-NYC WT+ fare, which is only pricing up on Matrix under IB codes, despite the whole journey being on BA metal. There's availability on the return TATL to upgrade to CW.

I'm guessing that unless I can create the fare on a BA ticket, I can't do this, but what about if I transferred the avios to an IB account?