>> So perhaps use Air Canada miles or United Miles?
1. it's often cheaper to fly from NYC to NRT via YYZ , then just flying YYZ -NRT on AC. Originating from YYZ, it's always cheaper to go through US. Randomly picked February, in J, it's $1000 - 1600 more to fly AC compared to UA/NH/JL/AA (~4000)
2. using "Air Canada miles" (aeroplan) to fly YYZ-NRT in J on AC = next to impossible to book + $650 of YQ
OP should post this question on Aeroplan forum. Did a few F aeroplan "miniRTW" rewards with a stop in Japan, on UA and NH.