From my experience, those are generally not blocked off to Diamonds. Recently flew from LHR-HKG on J. Was looking to change my flight from CX254 to CX252, and was able to see that all window seats were available in the mini cabin on the CX252. Unfortunately, wasn't able to change my flight, but I was able to reserve 12D on CX254 well before the date of travel. I only have CX silver status.