I think a walking foot is not what you need for this job. A walking foot is good for going straight, but if you want to make wavy lines, you would want to move the fabric from side to side, not turn the quilt as you change directions. You would want a darning foot with the feed dogs down so you could move the fabric easier from side to side, or wherever you want it.
I did the wavy line thing across the seam lines a long time ago. Just moved the quilt from side to side, making the loops from one inch to about four inches wide. I used latex gloves to get "sticky fingers" to do that. Still like that quilt.