Just did another search for YYZ-HNL-HKG-YYZ for a random dates in April and it got the "exceeds the maximum allowable distance permitted" message.
When I change HKG to SIN, I can get the following itinerary
April 15 - YYZ-LAX (AC791) LAX-HNL (UA342)
April 22 - HNL-NRT (UA879) NRT-SIN (UA803)
April 29 - SIN-LHR (SQ322) LHR-YYZ (AC869)
Tax:
U.S. Passenger Safety Fee $12.40
U.S Agriculture Fee $5.50
U.S.A Immigration User Fee $7.70
U.S. Customs User Fee $6.10
Nav Canada - Air Navigation Services $30.00
Canada Domestic/International Airport Improvement Fee $25.00
Canada Harmonized Sales Tax $3.25
Canada Domestic/International Air Travel Security Charge $25.91
Japan Passenger Facilities Charge $10.60
Narita Passenger Security Service Charge (intl) $5.30
U.S Passenger Facility Charge $5.00
Fuel Surcharge $250.00
U.K. Domestic/International Passenger Service Charge $61.10
Singapore Passenger Service Charge $17.30
Singapore Aviation Levy $5.30
Singapore Passenger Security Service Charge $7.00
Total per passenger $477.46