Actually, if it was marketed/ticketed as you say, there might be hope. In Shenzhen they might could have you do at least Customs (maybe Immigration also) then have you as CIQ-stickered passengers in Beijing, with agent on the PEK jetway to escort you through any remaining hoops in expedited fashion and over to secure international airside. Expect the worst case but maybe you'll be pleasantly surprised. Still doesn't solve the potential domestic flight delay issue though.