I can't really comment on BA but once I was able to select a premium seat without going through payment on LOT (even though payment was required) it stuck on CMT and I got the seat on the day of my flight as well.

I guess it's a layer the airlines build on top of Amadeus so if it's confirmed there I believe it should work. Again, can't comment for this case specifically but just wanted to share my experience as you mentioned confirming the seat and seeing it on CMT as well.