Injecting new compiled function implementations into a native executable during run-time?

Question

I'm implementing a Lisp-like language with a primary design goal of full interoperability with C. This means static typing, direct support of all primitives defined by the C standard, pointer arithmetic and low-level memory management. Macros are expanded by a virtual machine and the entire program is converted into C and compiled. This allows me to compile entire programs, but what if I want to be able to modify and compile specific functions interactively? Is there something I can use to integrate them back into the running executable without manual assembly hacking?