I would actually like to see someone make edible armor.
The Aqua Resin shouldn't have any long term stability issues. If it did, they would have a hard time marketing it since nobody would want to make something from it if it would just degrade and warp. I did some searching online and couldn't find anyone badmouthing its lifetime. What you will most likely experience is a lower quality of physical properties (e.g. it won't wet the fiberglass as well/thoroughly, it may be weaker or more brittle, it won't adhere as well or to as wide a range of base materials, etc.).
You can literally put anything in the resin to reinforce it, it's just a matter of how much reinforcement you get as a result. You could shred your sock and use it, if you wanted to. Fiberglass woven cloth, fiberglass mat, chopped fiberglass strands, and then all those things from other stranded materials like cloth, shredded twine, etc. The characteristics that you want to emphasize are materials with low elasticity, that will be wetted by the resin well. So you don't want to use a very tightly woven cloth, for example, because you'll have problems with the resin permeating through, but cheesecloth seems reasonable (I would even look for something a bit thicker, like loose weave linen).
Take extra care to reinforce stress points, like corners and edges, extra well, and generally make the composite thicker than you would have with fiberglass+polyurethane. Fiberglass's most common failure is cracking, and these materials will likely be more susceptible.
