But the answer to your question about affecting handling is: No, it will not, and in case it did? Then you could do as most racers do. loosen the floor pan bolts just enough to flex the chassis, and it will free up.
The only reason why I would think not legal is: If you hit something hard on the bottom and it shatters in 2 pieces it could fly into somebody’s neck or hands.
Your comment about seats being fiber glass does make a valid point
The only difference is that the seat won’t come apart into sharp edges or sharp pieces
Usually will take a slow process of cracks, and will most likely flex and bend, rather than sharp knife fiber glass body parts. It is a different composite of fiber glass that flexes more.
I hope this will help you more than confuses you, please feel free to contact SKUSA rulls, don’t just take my word for it. Fast Freddy. ;)