Water soluble thread would not work for washing the quilt top. After posting so much, though, it occurred to me that you might be able to avoid washing the top simply by airing it out for a few days - out of sunlight, though, to avoid fading. Fresh air might be enough to make it tolerable for quilting. Also, you could try putting it inside a plastic bag for a week or two along with several pairs of Dr. scholl's odoreaters shoe inserts with charcoal. The charcoal absorbs odors. These would be much safer options to washing - not to mention less work!