Client-server sounds more like RPC than FFI. That leads to SOAP and WSDL, or XML-RPC if you want something lighter weight.
The biggest problem I see with implementing FFI through a server is it's very expensive if the server needs lots of the client state. For instance, suppose you implement matrix determinant in C for speed. With client-server, you'd need to copy the whole matrix across the connection; with local FFI, the C code can access the matrix directly.
