Has anyone ever mixed fiberglas strands into drywall mud?
In a marine fiberglass-in-resin situation, the resin's surface tension pulls the fibers into the mass, with few protruding fibers. Plus, since the resin is of similar strength to the glass, sanding will remove both fiber and resin at the same rate, leaving a relatively smooth finish. 

With strands in drywall mud, however, the mud is likely to leave strands protruding. Plus, if you try and sand it smooth, you'll find that the mud is much weaker than the glass fibers, and you'll inevitably remove more mud than fiber, leaving a "hairy" finish.

People use fiberglass tape or mesh to reinforce the mud, but that only works if, when you're finishing, you only remove some of the mud but never get down to the fiberglass.