To stiffen fiberglass you have to use a resin/hardner that would burn out well below slump temps. Even if you somehow rigidized it, your form would be moving as well as glass you wanted to slump.
The only practical use of fiberglass for a slumping mold I have heard of is the use of borosilicate glass fibers in a plaster/silica mold to strengthen it and prevent cracking while firing. BTW, contacte me directly again, the email I sent to you bounced.