It's possible, but seems to me the long, tedious, difficult way to do it (compared to the standard mold/cast process). I guess it may be a little cheaper materials wise, but I would think the time involved in getting a clean, smooth, finished piece wouldn't be worth it (at least to me). The other thing is, by molding and casting you'll be able to produce multiple copies cheaply and quickly, which could help pay for the initial cost. When I first started my build I really tried to cut corners and do things cheaply, and I had allot of cheap, and less than "good looking" parts that ended up in the bin. I learned the hard (and expensive) way that doing something the right way the first time saved me from having to do the same thing again. That's not to say that there is only one "right way" to do things, or that you wouldn't be able to produce a good, professional looking bio using your method. Think about it this way... by fiberglassing over the sculpt, you're only saving yourself one step (the molding). I've priced out what it's going to cost me to mold the bio I've been sculpting for ages, and with the silicone, fiberglass, and everything else I need to mold it, I'm looking at barely over $100.00. That leaves me a mold that will produce a bio for me over and over (for the life of the mold) for $10-$20.00 worth of resin. I may be different from others, but my time is worth more to me today than the materials I buy, so it's not hard for me to spend an extra buck or two if it's gonna save me some time.
Brian