I once had success connecting in YVR without having to clear Canadian customs from an intl flight (ams) to a US pre-clearance flight (sea) on separate reservations and without a boarding pass for my onward flight. As I recall, at the transfer station there was a kiosk and a phone - for some reason AS would not issue a boarding pass to me via the app or kiosk but a phone call to AS from the transfer station solved the problem. I was honestly surprised it worked.