Kiefer, my advice probably won't help you but if you don't get good advice here you might try moving this to the Boyer's forum. The guys that build bows can probably help you.
Bows that have a fiberglass overlay like you describe are usually done that way to strengthen the riser and as such, it would not be a good idea to get rid of that. Many times those bows had a solid wood riser - meaning it was not made by laminating multiple wood pieces. If the bow is a low draw weight, it might not be that critical as it likely isn't adding a lot of strength and is mostly cosmetic.
I have reshaped the risers on several bows to fit my hand better, but never touched the fiberglass lamination on the back. Just be sure not to take off material in the very throat of the grip as that is the highest stress portion of the grip. I usually use a rasp to remove most of the wood, then a rat tail file to smooth, and finally ever increasingly fine grades of sandpaper.