Will not work.
Doubt they'll even let you board (with a RT booked like this, it will already look very suspicious) if the agents in DTW are having a close look at it (and quite rightfully, especially if you barely bring luggage etc.)
Even if they let you fly to PEK (technically allowed) you'll be found out. If you arrive from DTW, a boarding pass to DTW will definitely be catched by the immigration/boarding pass check. If you show them your boarding pass to a 3rd country, they'll let you through, BUT without the stamps from the check (and the picture they take from you at the check missing) you'll be singled out at boarding, and then you'll find yourself in quite some trouble (rightfully so!)
One guy who did a similar "stunt" got even into the airpland, then was removed from it. Absolutely deserved, actually.
Do NOT abuse TWOV. Personally, I hope everyone abusing it get caught. This isn't about not register at a place you stay overnight (or two nights), this is clear and outright cheating of the system what you've had in mind. And on top of that, TWOV checks will be further increased the more cheaters there are catched, so by doing so (and quite some people DID cheat the system early on, apparently!) you'd be making the life of people travelling on correct TWOV compliant schedules harder.