stand by in 1st
 

i have a milage ticket from BA for a flight from lhr-hkg in 1st class on CX, it is for the latest flight. If at checkin they have availability for an earlier flight, maybe even the 11:30 am flight, would they let me on?

I don't see why not.. just standby at the airport earlier, and they will let you know at boarding if you can get on the earlier flight.. would be more troublesome though if you have check-in bags, but sometimes they can confirm at check-in if the loads are really light..