Found this on AC site. It says available for international departures, so I assume if you can get there, you can get in. Guessing security should accept that since you have a valid reason to be in domestic terminal. More important to me now as schedule changes have made this a 7 hour layover

Which lounges are open

The following lounges are welcoming back eligible customers departing on domestic or international flights:

• Montréal-Trudeau International Airport, domestic gates
• Toronto-Pearson Internationa Airport, D gates
• Vancouver International Airport, C gates
• Calgary International Airport, Concourse C

Please check back soon for details about the reopening of additional lounges.