GLSL can't access a filesystem though? Any examples of how it would be abused or insecure?
Why would it be worse than Lua? GLSL runs on the GPU so to me it seems harder to access the wrong things even in case of a fatal buffer overflow in some implementation. Maybe I'm overlooking something?
And even if the language has support for 'bad' functions you can 'just' not include those in the implementation that mods can access.