Keep us posted. If there ends up being proof of delivery with that number, I will definitely go back to PayPal and raise a stink about it. Of course, that doesn't help me on the current auctions...

It worked! I entered the PayPal/USPS-generated tracking number on the Canada Post website and sure enough, it shows the item as being delivered today! I'm really happy about this. I don't know if this works for other countries, but at least now I know I can ship Global Priority to Canada and get tracking.
The only thing I noticed is that PayPal/USPS requires the recipient's phone number when you fill out the the info to ship. Not everyone wants to give out their number.