The did a quick hand layup of the rib yesterday using some carbon and basalt. It ended up weighing 178g and should be tough enough to stand on. I couldn't get any deflection by pressing on it in the middle, supporting one end on the bench. I didn't try really hard as this wouldn't be a good test as once its bonded in the shell, the shell will add more stiffness. Another reason I didn't try too hard is that the rib was not even close to fully cured.
I glued the rib in and could check out it's effectiveness today. First I put the part on the floor on some foam and stood on it. Definitely strong enough. Then I supported it on some blocks on either end and stood on the middle and jumped around carefully. My conclusion is that it is a bit overbuilt. It took about 50g to bond in so ultimately with a bit better layup using vaccum, just slightly less material the finished weight bonded in could under 200g. Now just to make at plug and mould so I can do one for the other side. Looking down on it from above.