No experience, but the Platinum Card rules (if that's why you are asking) indicate that accessing a NW lounge on a flight operated by NW is fine, without regard to the ticketed flight...

1. The Platinum Card® member must be traveling. Name on ticket must match name on Platinum Card. Platinum Card member must be 18 years of age for American Airlines Admirals Club® Lounges and 21 years of age to enter into all other Airport Club Lounges without a parent or guardian. Platinum Card member's spouse and children under the age of 21 or up to two companions may enter the club as complimentary guests. Code-share and affiliated lounge agreements do not apply. Special industry reduced revenue tickets are not eligible with Delta and American Airlines. The Platinum Card member must present his or her valid Card, government-issued I.D., and airline ticket to club agents. Complimentary access is specifically for the airport club that corresponds to the airline operating the flight (access pertains to the aircraft, not the flight number) except with respect to American Airlines marketed code share flights with an American Airlines flight number which are also eligible for complimentary access to the lounge. Note that amenities may vary among airport club locations. Meeting rooms may be reserved for a nominal fee. Card members must adhere to all house rules of participating clubs. For a list of participating airport club locations, call 800-525-3355. Partners and locations subject to change.