Depends on the airlines. As a Sapphire member you may be able to book BA-operated segments using Manage My Booking on BA's website; other airlines may or may not let you pre-assign seats, or some (I'm thinking Iberia here) may allow you to book seats but then promptly forget your reservation.

Phone AA and get your Amadeus locator, which you will need to access MMB on BA and/or to see your itinerary on www.checkmytrip.com. Then when you contact other airlines you'll have both the AA (Sabre) and Amadeus locators. I don't think either will work with CX, but not knowing who you're flying with, the question can't be easily answered.