It looks like you no longer have to "trick" BA.com into finding you partner availability when booking for Avios.
This morning I looked for flights from New York to Chicago and instead of making me jump through the normal hoops it just showed a newly redesigned page that showed no availability on BA and automatically showed me availability on American.
I tested this out NYC to Hong Kong, and even on a day with BA availability, it showed me a JAL flight right below that.
I've attached partial screencaps of each for your viewing pleasure.
This improvement has just made BA.com my number one airline site for finding OneWorld award availability.