I'm not talking about stuffing the roll bars tubes with cells, if that's what you suggest. It would be a[n angle-iron] box or boxes welded to the roll cage.
Basalt fiber:
deejaaa showed some samples. IDK if he reported any test results.
I haven't used it myself, but I follow developments at
Basalt Guru. It's similar to fiberglass and carbon fiber, but has interesting admixtures like
nano-glass coating so resin doesn't discolor it and
graphene additives to radically skew the physical performance.
It has interesting properties like low embodied energy because it's basically melted and extruded
rock.