What proportion of the Trent engine 777 fleet is directly under auspices of EASA?
I believe that it's 100%.
If the fix is a redesign of an engine certified part than the regulator for the manufacturer (EASA in this case) is the one that sets and approves the limitations and announces them to the world wide authorities to incorporate in maintenance actions. Of course the FAA would accept and announce them simultaneously
If the fix is a redesign of the aircraft plumbing, then it's Boeing and comes under the FAA and EASA would embrace these as well.