Update:
Checked-in at YHZ and there was the immediate vulture attack by the check-in dragon telling me that that my bag was overweight and I'd need to pay an excess fee; all with the grace of a low cost airline employee (I was at 27.5kgs-winter clothes!). Pulled out my ticket which I had printed out just in case and indeed the "American Airlines Luggage Rules Apply" was honoured fully; no questions asked. 2 x 32kgs. Thus Westjet may not be totally bound by its 23KG 'maximum weight' rules when another carrier is involved.

PS even though mine was an award booking it allowed me to pay for an upgrade to W at check-in! It was very reasonable cost so I did. Happy days with WS. That said I'd never pay to fly with Westjet after this experience- it's just too low cost for my liking. AC is a few notches superior for almost the exact same price.