What does "full integration" mean, and why couldn't that be achieved with some macros? He already showed how he used macros to achieve nice integration with LLVM (written in C++) for his interpreter.
Worst case, you'd have to write a name mangler, to teach your FFI about C++ symbols, but I imagine that would be at least an order of magnitude easier than an entire compiler (150K lines C++, 85K lines CL).
There's a lot more to it. You need to translate native objects to and from C++ objects efficiently. You need to work with C++ exception handling to unwind the stack without clobbering RAII.
Worst case, you'd have to write a name mangler, to teach your FFI about C++ symbols, but I imagine that would be at least an order of magnitude easier than an entire compiler (150K lines C++, 85K lines CL).