Use a TA and have him waitlist you for a NW J seat and put a similar UA confirmed routing in the same PNR. Armed with the record locator of the PNR, call NW and ask for a supervisor to invoke the Plat booking guarantee. One look at the PNR and the call center agent will definitely yell for its supervisor.

I've used this strategy to invoke my LH Gold booking guarantee. Just made a WL booking in LH C and had a confirmed BA C in the same booking.