No access to Polaris, or to the Intl Arrivals Lounge - that is just not going to happen, and I wouldn't waste your time trying. I would suggest asking Air Canada what options you have - other than Amex, there are no lounges with showers available in the G concourse - if your flight leaves from A, then AC might have a lounge arrangement over there. In my experience at the Amex lounge, the shower has been booked with a very long wait.