Why not just dump people who arrive from "safe" countries into the departure lounge? If they were in transit this would enable them to board another international flight without any checks at all (why does CBSA even need to check INT-INT passengers?). Then provide routes for them to either go through OSS if they want to transfer to Canada domestic, or CBP preclearance if they want to go to the USA, or an exit route for traditional immigration and customs if they want to leave the airport.

If someone arrives from a "non-safe" country, send them into the arrivals corridor like all international arrivals are now. If they want to go to CBP preclearance they can do that, if they want to go to another international flight they can go through security, or they can go through the traditional customs and immigration procedure, reclearing security once landside if they want to transfer to Canada domestic.

Am I missing something, or would this pretty much solve all the problems with the current setup? It would require minimal building work. OSS and non-OSS pax wouldn't mix and thus couldn't swap items. There would be no need to make OSS pax get on a stupid bus with terminals that weren't designed for OSS like YYZ T1, just have them do the DOM-INT connections path in reverse. There would be no need to waste money on airport workers checking boarding passes and CBSA officers checking pax who will never enter Canada. There would be no need to worry about whether foreign airlines are capable of issuing OSS-marked boarding passes or not.

Obviously this would only work at airports like YYZ T1 or YYC where domestic and international departures are separated.