My experience in WBC upgrades only happens for the NAmerica to Asia routes. What I've noticed is that NW's reservations does not release seats for B fare upgrades until the very end (last 5-6 days prior to departure). The Y fare upgrades usually map into a Z/C class which is usually available for sale.

A good indicator is to check how many available seats in these fare buckets twice a day (early morning and late afternoon). If the Z/C class tickets are still listed as wide open, then your chances of them releasing B-fare upgrades are good.

If you have time and don't mind connecting, press your luck connecting in DTW or at a gateway served by KL (look for codeshare flights). There may be a re-issuing fee, but if you get a WBC seat, then a small amount is worth a peace (piece?) of mind.