Try calling AC, I've delt with them before on flight changes and found them very helpful. If you want to talk to them today you coudl always try calling the Canadian number in Toronto.
I wouls not go the route of what mullking is suggesting and going with Air Transat in this case as you need the flight to be all with one carrier in the case of a delay on 1 leg you don't want to get stuck somewhere else!
One option that you might be able to try with them - and I'm not sure if this works - but Air Canada are part of the Star Alliance with several US airlines, perhaps they can book you on one of those? Alternatively, I know your direct flight to toronto was handy but there's always teh option of going via Heathrow and seeing what other direct flights they can offer you.
Good luck!
addob