> ... I suppose the interesting question is: if some of my dependencies are ported, and some are not, is there any way a normal compiler can call into a Fil-C-compiled library for just a few functions?

To my understanding, there's no way to do this: Fil-C is basically a managed runtime for native code, so the FFI implications are similar to those for Go or any other intrusive managed runtime. Which is to say that Fil-C could offer an FFI, but not without losing the blanket exit-instead-of-memory-unsafety guarantees it aims to offer.