I haven't built a fiberglass CNC anything.. but I have built a fiberglass airplane . My recommendation would be to skip fiberglass and go w/ Carbon. For the same thickness you'd be way stiffer than glass. You could also use Kevlar 49 [or is it 29?] in it as well, Kevlar is known for its vibration dampening properties, which would be a great asset on a mill or router. I'd build the basic structure out of carbon and foam, then just wrap it in kevlar [little if any cutting of the kevlar if possible as its a realy pain to cut] and then cap that all w/ either carbon or glass.
There's no doubt that it could be made strong enough. Alot of cars now have the structural support in their bumpers, made from glass. As well as the impact beams in car doors are built w/ glass.
It wouldn't be cheap however.. But if you were going to do PCB milling or light engraving and wanted a super fast machine.. it'd be the way to go for sure. Imagine 0-1000ipm in 1/4" inch.. that'd be some acceleration eh?
My 2 cents..
Jerry [someday I just might have to try this ]