To answer my own question, Qatar use the HIA lounge, directly on left and upstairs after security (but still not listed on their or explicitly on the OneWorld site).

At 07.00 the lounge was empty when we arrived. It is largish, open and airy with quite a decent spread of food, soft drinks and beer. Actually quite pleasant at this time.

That said security and immigration would have been a pain. Business class / priority gets you past the pre-queue into the main immigration area, but still a long snake from there. Security also slow with a long queue, only 3 machines working but again a priority channel on far left.

We booked a fast-track escort and it took about 15 mins from check in. A couple next to us who didn’t have an escort took a good extra 30 mins to get to the lounge (but don’t know if they went direct, of course). Overall worth the $28/person each way for an escort on both the way in and out.