there was a day when they said you can't build a stiff boat hull out of fiberglass it just will not work. I built a fly a fiber glass airplane and it is all about fiber orientation. A fiberglass mast is very doable . they were expensive to build and the resin technology was being developed at the same time as carbon fiber technology so now carbon makes more since. if they had not developed carbon fiber and had todays resins masts would be fiberglass.
I didn't say you couldn't, I said you shouldn't. A fiberglass mast on most boats very quickly runs into a material properties limitation.
................................fiberglass......al uminum......Douglas fir
Stiffness (psi)............1.2 x 10^6 ...10x10^6 .......1.8x10^6
Density (lb/in3).......... .055 ........... .1 ............... .02
So a fiberglass mast of the same dimensions would have about 60% the stiffness and weigh almost three times more. And when compared to an aluminium mast it would have about 20% the stiffness but weigh half as much. But masts aren't designed based on weight, that's just a necessary evil, masts are designed based on stiffness. So a fiberglass mast is going to have to be much heavier than either a wood or aluminium mast at the same stiffness.
Of course the RM(30) of the boat is going to go down with the heavier mast, so you may not need quite as stiff a mast as you otherwise would, simply because the boat will heel over further sooner.
The only boats I could see reasonably using fiberglass in a mast section are those boats that need radical flexibility of the mast. Something like a DN ice boat. But even there I am pretty sure they are using carbon designed to flex, not fiberglass. I know on fast catamarans flexibility is designed into the carbon laminate schedule.
If the fiberglass section is actually lighter than the wood, then one or both of them is seriously under built.