Bad bet on Korean Air suggestion, IMHO
Regarding this:
"Do you think it would be feasible to take the KAL non-stop and just not enter a FF# and then hold on to the BP / ticket until NW is part of Skyteam and then ask for credit for the flight in WP? Or would this get negged as a flight having taken place prior to the partnership commencement?"
No, I would not do this.