William, the dealer from whom we got our giant quilting robot, sent us a video a few months ago of a new technique he'd invented: Stuffing thick yarn through the center hole of the hopping foot and letting the machine stitch it down to the quilt.
This works amazingly well, though with some limitations (no breaks in the pattern, and no sharp corners).
I'm not sure what we'll use the technique for, if anything, but it's really fun that it works at all. I did four experiments. The first one, the Pi quilt pattern, was a failure. Almost tore the fabric. Then I made a test pattern with only smooth, gentle curves and no breaks. That worked great.
This style of yarn seemed to work best:
This kind tends to get bunched up and look sloppy sometimes:
And this kind just always look sloppy:
All the yarn I tried was super-think, because I feel that it has to be thick or the needle is likely to miss it. There is nothing holding the yarn in the proper position other than the fact that it's as big around as the hole in the foot, so it can't really get out of the way of the needle.
A real couching head would ensure correct placement of the yarn and thread, but it would also cost money, and doesn't currently exist for this machine. This technique, on the other hand, costs nothing (though I might decide to bend up a wire coat hanger to make a guide for the yarn).