Yes, I have had this happen a couple of times on Friday morning flights out of YYZ, Terminal 1. When this process is in place, you can't enter the line for US immigration clearance directly (unless you have Nexus/GE). Instead, they make you wait at Area J in Terminal 1. They have several lines in that area, depending on when your flight leaves. They let people out of the lines at Area J to go to the line for US immigration clearance room about 1.5 hours before the flight time. If you are late coming to the airport, i.e. less than 1.5 hours before flight time, they let you through right away to US immigration.

I fly out of YYZ quite regularly, usually on Fridays, but sometimes on other weekdays. I have only seen this process on Friday mornings. Not really early, but say, starting at 6 or 6:30 am. I have arrived at 5:15 am on Friday morning (for an 8:30 flight) and didn't have to go to the separate waiting line in Area J. (I like to get to the airport early in case anything goes wrong, and I also like to have a drink/snack at the Maple Leaf Lounge since I am an Aeroplan Elite member.)

One time I arrived at 6:40 for a 9:15 flight on Friday morning, and had to go to the lines at Area J. They wouldn't let my line (for 9:15 -9:30 flights) through to the line for the US immigration clearance room until about 7:35 or 7:40. Once they let us go to the US immigration room, it took another 45 minutes to get through US immigration and security. That left us about 15 minutes before our scheduled boarding time. It was extremely annoying to arrive early at the airport, only to be required to wait for an hour before I could even enter the US immigration clearance room.

Hope this helps.