I believe he could replace comma operator with AOC (Any Other Character) operator: just replace comma with "something" which is not yet used. Choose some fancy-looking Unicode character that no one will really use but the compiler itself to generate intermediate code.
If you don't expect it to be written in code, then there is no reason at all to even defined what char is used because the compiler doesn't internally handle code as text. You can just define it as an AST node and be done with it.
