I don't believe you'll be able to reroute through N/A. I looked up your fare and found the routing rules:
SLBB1Y:
Code:
MEL-LON FRI-08FEB08 BA
TAXES/FEES NOT INCLUDED
ADULT FARES
CX FARE FARE C AP MIN/ SEASONS...... MR GI DT
GBP BASIS MAX
1 BA 682.00R SLBB1Y S | 17JAN4-24FEB4 R EH
SPECIFIED ROUTE: 0402
MEL(QF/BA)BNE/CNS/MEL/DRW/SYD/PER(QF/BA)SIN/BKK/DPS/JKT/HKG/SHA/
MNL/TYO/BJS/BOM(QF/BA)SIN/BKK/DPS/MNL/JKT/HKG/SHA/TYO/BJS/BOM
(BA/QF)LON(BA/QF/AY/IB/EI)FRA/PAR/ROM/ZRH/HEL(QF/BA/AY/IB/EI)
FRA/PAR/ROM/ZRH/HEL(QF/BA/AY/IB/EI)LON
(BA/QF)LON
MEL(QF/BA)SIN/BKK/DPS/JKT/HKG/SHA/MNL/TYO/BJS/BOM(QF/BA)SIN/BKK/
DPS/MNL/JKT/HKG/SHA/TYO/BJS/BOM(BA/QF)LON(BA/QF/AY/IB/EI)FRA/
PAR/ROM/ZRH/HEL(QF/BA/AY/IB/EI)FRA/PAR/ROM/ZRH/HEL(QF/BA/AY/IB/
EI)LON
DPS/MNL/JKT/HKG/SHA/TYO/BJS/BOM(BA/QF)LON
There's not a single North American aerodrome listed amongst that lot!