"I'd never heard of maxi piping and certainly it wouldn't be available in Hong Kong (few people sew for themselves here!) so I thought about what might do the job. I went to an electrical shop and just felt the various wires (cables). One for computers seemed to have the right thickness and bend-ability so that's what I bought. They thought it was very funny that it was for sewing!" continue
You can also fill a bias strip (for piping) with any twine, string, yarn, or crochet thread. Think about how you will want to clean the finished item. If you use any version of "wire", it could interact with the cleaning fluid or soapy water to cause stains on the item. Wire can also be too "rigid" for a nice look when you're done with the project. And, for those of us who have been gouged by an underwire from our bras which has broken and then pushed out through the casing it is normally housed in, the prospect of a wire breaking in the piping and tthen sticking us is unpleasant. If what you want is a larger (for visual impact) "line" of color, consider inserting an uncorded piece of bias in the seam. Just cut the width you desire, fold it in half to press a nice crease, lay the pressed strip along the seam line so that the fold extends into the garment (beyond the seam line) the desired distance, and sew the seam. When you're finished, you'll have the emphasis where you wanted it and you won't have cleaning or injury worries!
Hey, in thinking "outside the box", it also occurred to me that you might find some commercial fishing line "plastic" or nylon that would give you the stiff effect while avoiding the metal problems of wires.