Though I am far from an expert on the subject, I can certainly confirm problems with ethanol fuel damaging fiberglass marine fuel tanks. I am a boatbuilder from NJ(with required all marine fuel to be 10% ethanol minimum) and I have seen the effects. Marine applications are further handicapped by the fact that these fiberglass tank equipped boats tend to suffer from both of ethanols great weaknesses. Boats are typically around high humidity envoirnments ...obviously. Ethanol naturally attracts and accumulates water through vents, etc. but does not readily mix with the fuel. So water gets in the tanks, and stays seperated. The other major contributating factor is that most boats do not get run too often, allowing the ethanol to attract more water.
The other major issue is the ethanol in your fuel wants to break down the fiberglass resin itself. The water collecting issue is a logistical hurdle to deal with, and there are a good number of people out there who are flying on nothing but ethanol blended fuel(though mostly in planes with metal or polyethylene fuel tanks). But in your instance, the ethanol wants to destroy(soften/break down) the resin in your tanks (vinylester resin appears to be most ethanol resistant). The softened resin then gets pulled into your fuel lines ...and bad things happen.
http://www.boatus.com/seaworthy/fueltest.asp
I have heard mentions of special slosh coatings that people were trying in an effort to "seal" the ethanol from the tank resin iteslf, but I have not heard of any reliabe information that suggests that it works satisfactorily. One that comes to mind is KBS coatings. To be clear, I have NO first hand experience with this stuff so therefore I have no intention on recommending it in any way.
http://www.kbs-coatings.com/Gold-Standard-Tank-Sealer-Information-Instructions_ep_39-1.html
A quote from a KBS company rep regarding their product(on a boating forum):
"When it comes to prepping and sealing the inside of a tank, you MUST USE the prep products and use them correctly. Also, if a tank already has substantial damage from ethanol blended gasoline, the cleaning and drying process is critical. Fiberglass can be rather porous and will hold moisture and/or fuel residue. That's why you need to use the AquaKlean degreaser correctly and possibly multiple times to remove all the oily residue from the old fuel. Rinse completely and allow to dry thoroughly.
Proceed with the RustBlast which, although there is no rust in a fiberglass tank, adjusts the ph to a more acidic level. Just a quick slosh with the RustBlast, pour it out, then rinse thoroughly again with clear water.
Now again let the tank dry thoroughly, as you do not want to trap moisture under the sealer. Use a blow dryer or shop-vac on blower to keep the air moving inside the tank to help the drying process. We place the tank out in the sun, plug all inlets, and rubber band a piece of plastic wrap tightly over the largest outlet. If after a couple of hours, you see moisture forming under that plastic, you still have some drying to do.
When you're sure you're dry, stir up the sealer and pour it in the tank. Slosh it around being rather methodical as you want to be sure to coat the entire interior surface. Any exposed fiberglass can still be affected by the alcohol. Once you're sure you've coated everything, prop the tank up and allow any excess sealer to drain out. You do not want the sealer to puddle inside.
Now, allow the sealer to cure at least 3-4 days before adding fuel again. Heat does not cure the sealer any faster, and therefore placing the tank out in the hot sun will not help, and may in fact hamper the cure strength. Just let the open tank cure slowly.
This is not a one night project. Take your time and you'll get great, long lasting results.
These directions and more can be found at: www.kbs-coatings.com"
A bit more drastic, but I have seen a few boats cut an access hole in the tops of their tanks just large enough to insert heavy duty bladder tanks. This basically turns the fiberglass tank into a protective case for the self contained bladder tank. This is certainly not the ideal solution for an aircraft, but it apparently was considered worthwhile for a few folks who were looking at $50,000 plus to replace their huge built in fiberglass tanks on their 50+ footers.
If the option exists, and you have a fiberglass fuel tank(s) built into your aircraft, the safest bet would be to run 100LL only.
George