I just made a pair of fly front pants out of stretch corduroy. It's a difficult thing to get a nice, smooth fly topstitching line while trying to avoid the zipper stop and sewing over the bias or curved part of the topstitching. I just cut a piece of blue painters' tape using the topstitching guide as a template. I stuck it in place and sewed at the edge. It provided a nice guide and also stabilized the fabric.
If you use this method make sure you do a test on a scrap to see if the tape is compatible with your fabric.