What you are refering to is called 'mud glassing'. Tandhem posted a tutorial here and he made a youtube video here demonstrating the process. I haven't watched the whole thing all the way through, but I noticed he is not using a respirator which is a bad idea.
My understanding is that you want to apply the fibreglass after the first layer of rondo goes in but before it hardens.
I tried that for my thighs that I rushed to finish. It felt more solid and less prone to cracking in the areas that I tried it such as the large panels along the sides. However, since I didn't use it for the whole piece the edges were very weak and have since cracked or broken. In other words it really only seems effective if you overlap the fiberglass cloth over most of the piece in which case you might as well just go resin.