Don't know if this will work, but how about booking your ticket online YYZ-BKK-YYZ (or just putting it on hold), and then calling AC to change the date on the BKK-HKG or HKG-YYZ segment so you have your 10 day stopover. If they won't do it, you can just cancel the ticket assuming you do this within 24 hrs of booking it.

If you explain to AC that this is what you wanted to do, but could not do it online due to multi-city limitations, they might help you out.