It's reasonably easy if you can treat the Safe Rust and Fil-Unsafe-Rust code as accessing different address spaces (in the C programming sense of "a broad subset of memory that a pointer is limited to", not the general OS/hardware sense), since that's essentially what the bespoke Fil-C ABI amounts to in the first place. Which of course is not really a good fit for every use of Unsafe Rust, but might suffice for some of them.