I created 6000 trees and looped every 0.1 second through all of them and compared their current hp to a saved value. Its quite resource intensive, in the otherwise nearly empty map was no noticeable /fps difference though. Note that I was using an optimized JASS loop and not GUI.
This does not give you the "attacker" though. For this you would have to use fake destructible units, as said above. And 6000 unit on the map is a no-go..
Just catching orders is not that good either. First its just an order, so is not directly related to damaging a tree and second a normal peon can start harvesting on different orders.
Its hard to say what is best for you, please specify what you want to do exactly.