Sorry: as hillrider pointed out, you will not get this info on an AA codeshare flight, operated by RJ. Guess the only solution, as others have said, is to call AA for RJ locator, and then call or email RJ for seat assignment. Actually you could just email RJ with your name, date and flight number, and they can find your record; ask them for seat assignments at the same time. I have had quick response from RJ for seat assignments using:
[email protected]
Happy Travels.