I did this 2 or 3 years ago: SEA - YVR - SEA - PEK
I can confirm you have to clear customs and go back through security. The US outbound flights are not the same access point in YVR due to preclearance.
I talked to several airport staff abiut my crazy connection back to the US, and even mentioned it at customs. No one mentioned anything about cabbage... Or cabotage, or whatever. They just thought I was crazy.
Saved $3k on a business class ticket to Beijing, that's not crazy!
I connected thru very quickly, but gave myself 4 hours to account for potential delays.