The problem you run into with Fiberglass is that if it is not mixed perfectly it can become very hard and almost brittle. I mean, imagine it as actual glass. It can chip and crack and is quite a pain in the ass. So if you have a large caliber rifle the recoil will destroy that thing. If you use the mixture at all it needs to have a significant amount of the fiber screening to go with it. The issue that arises is that fiberglass, although incredibly strong, is also quite heavy. The finish is much different than you would expect for the final outcome for a rifle stock. Like I said, I would look into something more like a urethane expanding foam resin, polycarbonate, or some sort of casting plastic resin. Check out companies like Smooth On, Devcon (yes the bedding compound), or Goldenwest Mfg. You can even google search plastic resins and find a dozen companies that specialize in this sort of thing. There are so many plastic composites that are ridiculously strong with minimal weight. Again, think about the Hollywood movie sets that build a 1,000lb boulder that actually only weighs 20 lbs. Like I said, after having worked with Fiberglass, I don't think I would use it as a filler, or outer layer to my stock after having worked with it.

These plastic resins/compounds are so strong that you wouldn't need any sort of metal framing. However, the aluminum action bedding would certainly be a significant improvement to the stock. I would suggest pouring the stock and allowing it to cure, sanding, carving and getting your shape done. Then, router out the barrel and action channel. You can epoxy bed the aluminum frame into the rifle stock so it is permanently part of the body. There are tons of ways to do it. Let me know what you're thinking. I don't think this is out of reach, you just have to figure out a resin/plastic/compound that has a good strength to weight ratio.

Keep me posted!