Is this shower stall/bathtub against an outside or inside wall? If against an inside wall, someone could open up the backside wall and install wood blocking attached to studs (that reach out to the back of the fiberglass shower/tub wall then the grab bar can be screwed to the fiberglass wall (catching the solid wood) with minimal disruption). Fixing drywall is easy.
If against an outside wall, check out stuff like this
Install grab bars in fiberglass showers with The Solid Mount