As a frequent 1P/*G traveler to YYZ, I have had no problem on UA, US or AC using lounges on Y tickets USA-Canada. I've used lounges at LGA (US and UA), ORD (UA), PHL, LAS (US), and YYZ (AC) by presenting my boarding pass (UA) or boarding pass and *G card (US, AC). Never had anybody say anything besides "enjoy the lounge" (although even that's rare! ).