Fascinating project! I kind of hope to need to work on C++ sometime so I can try it out. Particularly the Clang AST library would be a enormous boon for any LLVM / C++ / C work. IMHO, 9/10's of any compiler work is just figuring out how to write recursive functions to modify the AST which is a pain using C++ and visitor patterns. Working in LISP for these parts while being integrated with C++ for other parts sounds like it would be awesome if not outright fun.