I think any form of protection is good, but you couldnt do it prior to fitting as you have to weld the metalwork to the car and itself so the zinc may cause problems.
I have no idea how the fibrefill/fibreglass matting/resin would stick to the zinc coating though, so would be slightly concerned at using it all over the frame prior to bonding.
I am removing as much of the light surface rust as possible & ensuring all remaining exposed metal or welded joints are covered with hammerite. Provided the surface is clean when painted, I think this will be sufficient to keep the rust at bay.
The tube metal is way thicker than any of the MR2 chassis metal & that only has a thin paint layer & has had over 10 years head start on the rusting process compared to the bits we are adding.