Visit the OneWorld website (
http://www.oneworld.com)
Click on "Getting There"
Left Hand side, view the Airport map of Bangkok.
Click on the terminal to show finer details.
They discuss a "QF/BA lounge" at exactly the location I remember it (Across from the CX one).
Not sure why BA doesn't list it on their site. As discussed above, the JSA is in effect for BKK so BA and QF share everything.