Even that way will be very difficult to make it work.

The Fil-C side could change the data behind the pointer, which will not reflect on the Rust side due to the copy. Even if you manage to copy back the changes, Fil-C could also persist those pointers in e.g. global memory: at that point you no longer know when it's safe to copy back (or forward) any change.

The only way I can see this work if you cannot pass any Rust-land pointers into Fil-C, but at that point you could also compile the C code to WASM and use the WASM FFI (which has similar restrictions)