@ai6yr i've used rope, paracord, and even dacron line for piping dozens of times without issue (it was a middle-of-the-night sewing project and spare line from my parafoil was the only thing on hand).

so long as it fits your presser foot or you can rig something to get a consistent stitch, i don't see why it matters what you put in there so long as it's uniform diameter, bends easily, and doesn't snag.