Originally Posted by
cfischer
how does an AF C-classs mix with NW Y-class on 1 PNR

Well, I can tell you that NW is not readily accepting BPs from connecting flights and NW Y-class only gives access unless you are FB GE aor PE.
Actually, I have just that type of itinerary coming up in a few days. There are many reasons it could occur: one-class regional jets, corporations telling their travel agent to book the transatlantic sectors in C and domestic in Y, award availability, etc...
Regardless, all you need is a same day international Skyteam boarding pass, so the AF C-class BP will suffice. See
http://www.skyteam.com/EN/benefits/skyteam_lounges.jsp
"All international First and Business Class passengers travelling on a same day international flight operated by a SkyTeam member have access to these exclusive facilities."
Enjoy!