I can't see why not. Why fiberglass, do you need more rigidity than you might get with rubber? What scale are you working on?

I've used fiberglass to reinforce a mold, but the face of the mold was still rubber to allow easy wax release. I'm not at all sure how well wax would release from fiberglass itself, though.

Just release it REAL WELL and there should be no problem at all. I would use a silicon spray for a release as it seems to work very well when wax slops over the mold onto the fiberglass.

Absolutely and I have used it many times.  It has both advantages and disadvantages though.  If the piece has no undercuts it is fine, if it has only some and the fiberglas mold can be constructed in more parts to lock but still remove without undercuts it is still fine. That may eliminate many forms though.  You do have to use a new parting spray with it every time you use it and not depend on the last time it worked it would again.  For that I found two options.  A commercial silicone spray is the most common but will leave a wax surface with a mini amount of it on it so additional spruing or wax alterations get harder unless you also wash that back off the wax.  For that I would use a paint thinner or even lacquer thinner but caution, it will start to soften the wax surface again if left on too long.  The other option is to use a liquid soap parting agent on the wax. Dilute with a minimal amount of water, or better yet some alcohol since that will then evaporate back off much faster.  This method requires a pause and drying time between each wax pour so no water is left on the mold.  The soap will also have to be washed back off the wax when done but that can be done easy with just more water.    Finally be sure to pour your wax into the mold when it is as cool as possible and still not leave any rings or ripples as it cools and do it with a warm or at least room temp fiberglas mold so it does not chill the wax to fast.  

There is loads on this subject in the artmetal archives.  We did a wax chat some time ago that gets into this real well and it is at:
http://www.artmetal.com/village/chat/main/transcrp/980423MN.htm
Worth a peek. Also hit
http://www.artmetal.com/search
and put in some wax related words and get a whole lot more. It is fun stuff once it is under  control and is unlike any other media I know of.

If you do a good search you can also find out how to pour waxes into the same wax molds made up by controlling the temperatures of each.  It is one of my most used systems for hand held sized duplications.     For full human figure sized duplications though I usually go the longer route with urethane rubber molds backed up by either a plaster/fiber  or  fiberglas mother mold.   Enjoy.   bpfink    

We have used fiberglass a couple of times as the mother mold for a quick process. It is lighter, thinner, a bit easier to handle but can be sharp on the edges. Be sure to grind off all the sharp points.

Tim Costello
Zingology Manufacturing
Fine Art Bronze Foundry
www.zingologymfg.com