Hi! I just used a dynamic offset to load the Pokecenter attendant script that supposedly fixes your save in the compatibility patch. It clears flag 22F, if 22E it then fixes the save by removing some item (removeitem 0x35 0x1, DEC_ENOTEMAT? not sure what that is), adding some fossil flags if necessary, and adding the GBSounds/Running Shoes if necessary (I don't know if this is all that's needed to actually fix a save). I compiled that into an Unofficial Throwback ROM and loaded my vanilla FR save. I was able to talk to the attendant and get the save 'fixed', and I checked that the GB Sounds item works as expected. Then I activated Mystery Events and talked to the new NPC in the pokecenter and correctly received the event items. I then walked around a bit and fought a nearby trainer. All of this worked normally, though it was quite a short test.
When you were checking the offsets, did you find any other places other than the attendant script that were in conflict? I'm not sure that's all there was to the patch. My 'imported' save seems to work fine but it might blow up later. Please bear in mind 5 days ago I didn't know ROM Hacks where a thing, I've been trying to gather information to experiment a bit since your answer but I might be spewing bs in my ignorance.