A Lisp macro is simply a function from an AST (in s-expression form) to an AST. Other than that, it's an ordinary function and has full access to the power of the language.

Implementing C as a parser that builds s-expressions followed by a collection of macros to translate it into Lisp is certainly possible; I did it in 1984. I wouldn't say it was trivial, though; nor was the performance anything special (even then).

I will definitely take a closer look at Graal and Truffle.