Wait for NW's Valentine's Day Europe sale (if they have it this year), and change one of the BOSs to AMS. You'd still have a few dollars left over to buy 2 or 3 smartsnacks.
I actually think you'd do better with MCO, TPA or SJU or whatever prices out from LAX. More miles than BOS and you can get upgraded to Mexico and Caribbean.
You could actually book two back to back KIX fares. ORD-KIX x 2. $1032 all-in. 28,400 EQMS plus 18,000 bonus miles from promos. Out 3/2, back 3/5, out 3/9, back 3/12. Would cost you 4 vacation days, though.