I am wondering the same things Terry is.
I considered putting fiberglass inside of my new set of TE2 armor. I had tested it on a hand plate, and it seemed to work well, but when we got it into one of the thighs it really didn't do anything for it. The benefit (strength) was far outweighed by the negative (mess, time, cost, added weight, etc.) Plus it pulled right off the HIPS — thankfully — very easily so wouldn't have lasted long anyway.
There might be ways of making it stick... and I know you're talking about armor that's completely made from fiberglass (not "coating" a set of armor), but I just have to think what Terry said is right... there are plenty of good, affordable options out there so, to put it bluntly, what's the point? And I ask that because I want to know... in case we're missing something.