NW has offered 60 day "trial" memberships in the past.
I purchased one last summer for $85.
Just go into a WorldClub the first day of your travels and purchase.
This will make sense if you are going to continue your mileage run for elite.
It has previously been reported here that one-day passes are not good for the entire day but are only go for one visit at one airport. I do not know this for sure but it has been reported previously in this forum.