Are there any examples how to force C/C++ libraries within a Rust build to use Fil-C instead to improve security? Is it just a matter of overriding CC/CXX?

Won’t work

Can’t link Fil-C code to regular C code

And rust uses regular C ABI

You could make it work, if you teach Rust and Fil-C about each other. Nobody has done that (to my knowledge)

This would be a fun and popular project for the right sort of person