Lightweight Harleys.....? Blasphemy! Then you'd have to call them something other than Hogs.
Seriously though, all things carbon fiber are not necessarily the best. The trouble with cf and resin for fenders is that, although they can be incredibly rigid and light, the fender may not hold up well to rocks thrown by the tires. I've thought about this subject a lot in an effort to get an FXR to under 500 lbs. with cf fenders, and tank. I've made sailboat parts with cf and, for that, it was perfect. Light and stiff. You never see cf parts on motocross bikes though, even the factory bikes. They're all fairly soft plastic to take the abuse thrown at them. I'd be leaning toward Kevlar, which is used in bullet proof vests, at least as a barrier layer to stop the rocks. Maybe a Kevlar/carbon fiber combo just for the fenders. If you've ever seen an all cf Indy car hit a wall you know that strong side force impact turns that cf stuff into tiny little black butterflies. My old '69 Norton Commando S had a fiberglass tank with metal flake and color embedded in the gel coat. Strong and fairly light. The fenders on that bike were chromed steel though.
I have a pretty cool ABS Harley front fender that was made for Sportsters. An actual factory accessory. It's not real rigid but it looks like it would hold up to rocks just fine. Rear fenders have to bear a lot of weight and I doubt ABS would work out there. Still, I think it would be great for someone to try applying the latest composite materials and engineering to the fenders, tanks, bags, and fairings. A 650 lb. bagger would be very cool indeed.