I have used batting along with wash-away thread for machine trapunto work, but have not heard of using tearaway/washaway interfacing for this. How would it be used?
What I was taught with the batting is to use a high loft poly. Machine sew the top with high loft poly underneath, using washaway thread for the pattern. Cut away high loft poly outside the trapunto design, about 1/8" away from the stitching. Layer this top with batting and backing in traditional way, then sew around washaway thread lines with regular thread.