This would be something like Cardinal Components' PlayerSyncCallback. This is needed because if you want to sync data to the client from ServerPlayerEntity to the client's PlayerEntity you either have to use Mixins in a lot of places to catch all sync events or sync every playerTick which is inefficient. It would be usefull to just be able to hook into a event.

Yeah, this is a good event to indicate mods to sync player info to the client. On a side note, I wonder if these events should have more parameters indicating if all attributes should be sent or just changed attributes need to be synced (e.g. changes in dim may need a full send while others can just have partial)