I tried both lounges on Nov 30. All I needed was to show my boarding pass (showing my NW PE-ST E+ status) when I walked into NW Lounge. Small but adequate. Not the best in the world. Quite boring in my opinion. I got bored so decided to try a KE Lounge. I went to the one (West) close by NW lounge (KE has two lounges: one in West and the other in East). KE Lounge receptionists requested my name, NW WP number, and flight number. As easy as that, then I was enjoying much bigger space, free internet, and more Espresso in KE BC Lounge.

KE BC lounge is also located on the other end (East) for primarily KE passenges. The West Lounge is primarily served for those BC passengers on other flights serviced by KE ground personnel. The lounge is much roomier. Free local calls, internet, some snack, etc. I didn't explore enough to check a shower facility or sleeping room.

To answer your question, you are not able to access KE FC lounge unless you are flying on KE First Class according to the rule.

Von Boyage.