I just went through this in January. The BP scanners rejected our BPs and the CATSA lady told us we had to go to international. We said we understood the flight left from the D gates and we needed to go there eventually, but we had several hours to kill and wanted to hang out with a friend in the domestic side for a bit. Eventually she relented and let us through, but the machine definitely didn't want us there, and I could see this being a YMMV depending on who's working CATSA that day.

I do wonder whether clearing via the VT checkpoint upstairs might make this easier, since I think there's just the one and maybe they're expecting to accommodate both domestic and international pax (while my wife and I both have NEXUS, our FOTSG friend didn't, and we made the mistake of going through C security with him).