A guy on another forum mentioned that he was going to stiffen and increase the draw weight of a PVC bow by running fiberglass strands from one end to the other, inside the PVC, and embed the 'glass strands in epoxy -- pretty much what we do with sinew on a bow's back, but inside the formed PVC.
I haven't run into the idea of loose strands of fiberglass. I've always seen unidirectional or cross woven strips, sheets, or the usual 'bow glass'.
Some PVC bowyers I know stiffen/ increase the power of PVC bows by using two or three length of fiberglass rod (driveway marker rods) inside. But design wise you're limited pretty much to straight bows or Magyar designs with siyahs on the ends of straight limbs.