not to burst any bubbles here or anything but this type of procedure isn't really new.
Working on my Boba Fett suit I stumbled accross it for reinforcing the guantlets.
Oh and about "parts too small to get fiberglass mat into" the way to solve this is using a paintbrush, or sponge brush, and very tiny pieces cut up prior to adding the resin you wet the part, put the mat down, then wet the brush and dab the top into those tiny spots, although this has very limited fuction for sizes smaller then a quarter due to how thick the mat is itself. Using a literal Puzzle Piece method to slowly resin the inside has the best results - typically only working on a "flat" area no bigger then 4x4 inches and complex areas of 2x2inches, this way you mix a very small amount at a time, give yourself plenty of time to work it flat (to avoid having to sand a ton) and paint/spongebrush the top to aide in further reinforcement, smoothing, and pushing the mat down into smaller spots and corners your finger may not want to fit.
Bondo was meant for auto-body filler, and thus works well on large, semi-flat or easily curved areas, anything small/tight (like most the things we wear) it destroys the detail due to it's very sloppy slimey sticky properties, and always, no matter what you use, it will require sanding, if only just a bit to get it to hold paint better, up to days of sanding all the cuts out.
Although points for creativity on this I guess. Reinforcing parts with mixes like these has been around since before a lot of you were born.