Soak the derailer all you want. Just lube up the pivots with your lube of choice after all is said and done. As for the freewheel, I dont know for sure but I would personally blast it a few times with some simple green and scrub it with an old tooth brush and rinse it. Not much else you can do to clean it. Just dont blast it with a power washer or anything of the sort and all will be fine.