http://www.nwa.com/worldperks/elite/...l#LoungeAccess
If you interpret the section below literally, as long as one is a NW Platinum on a SkyTeam-operated international flight/itinerary, one should should have access. YMMV, of course.
Lounge Access to SkyTeam member lounges on valid itineraries
Lounge access is available to SkyTeam-member lounges for First/Business Class passengers and SkyTeam Elite Plus members booked in Coach, flying on or connecting to/from an international flight operated by a SkyTeam airline. Lounge access must occur on the same day as the required flight.
To access the lounge, you must present your boarding pass for your same-day international SkyTeam-operated flight and your WorldPerks Platinum Elite card. A WorldClubs membership card is not an eligible credential for SkyTeam lounge access. SkyTeam Elite Plus members may be accompanied by 1 guest.