The thing is, if you use the wisp harvesting, your unit will instantly be in the position of the tree, and for other harvestings, you'll have to switch the animation to the dark magic animation which i've tryed and couldn't find the animation, but then after that you'll have to make a dummy unit that spawns at the position of your harvesting unit in order to recieve the lumber, make the dummy unit invisable with locust and invulerable.
Something like that, the dummy part is going to require triggering that I don't know since harvest doesn't seem to work for triggering.