Thanks for the reply. I was expecting the answer to be "no", as I couldn't find any info regarding the IATA Baggage Resolution applying to carry-on luggage and I'm sure if it was explicitly mentioned there it would be easy to find.

However...

...before seeing your reply I spoke with someone from American Airlines who stated that the AA carry-on allowance would apply to the whole journey...???

I hate speaking with airlines and getting a completely conflicting answer every time I speak with someone different. The level of incompetence is shocking...