To get to the international lounges, you would need a ticket to an international destination (besides the US).

Even if you bought a refundable ticket to some other destination, you would then have to re-enter Canada after visiting the lounge, head to the US departures area, leave Canada again, and then enter the US.

So, even if technically feasible, not worth the prospect of a foul-up with Canadian immigration or the time you would need to waste to refund a ticket and go through Canadian immigration 2 extra times.