I am not very familiar with the inner workings of your addon, but I think you can still make it work as an opt in solution. At least for boss fights, determining how many percent of damage a player himself did should still be possible, even after the change. You can then either let the user share his data after the fight via chat, or do some magic and send it via map markers or something.
Normal mobs may be a bit harder to calculate as you would need to somehow get their overall max hp and how much healing they did to get accurate percentage of damage done.
It might not be as wholesome as before, but you can still see if you are doing good or not in a boss fight, even when no data gets shared by other group members.
