There's no AA lounge in the transborder area. Only AC or AS ones (no reciprocity with AA unless you're an AS lounge member). The BA and CX lounges are in the international departures area so you can't get to them from the transborder. The BA one does not open until late (since the BA flight to LHR leaves at 2030).

FWIW, if you want that extra few miles, can you fly HKG-YVR on 838/839 if operating or on the YYZ flight and then switch to 888/889? That's worth an extra 800 basic miles.