That seems like one of those things that ought to be inherently contradictory, but I'll be interested to hear how it goes. Note that fiber fillings in a material inherently reduce its layer adhesion strength, so your formerly stellar adhesion may not be anymore with the inclusion of the glass fiber. It'll also make your flexible TPU a lot less flexible. This mitigates the two major advantages of TPU, which leads me to wonder if it's even worthwhile. But still, give it a go. For science!

Don't forget to get a hardened steel or gemstone nozzle, otherwise you'll destroy your brass nozzle in very short order printing with fiber filled materials.