Does that even compile? Thought you'd need "if('/'==..." Well, I don't know C much...
In any case, that breaks documented behavior. We *had* something like that in, for a while - checked for '\' *or* '/', actually. Took it back out, because it broke existing code.
The documented way to do this is not "%include "\bar.inc", but "-I bar\" ('\' or '/'. as your OS requires).
The beauty of Nasm is that you can run your own "private forked version" if you want to. You won't be the only one! But Nasm works, as documented, as it is... Nice work, just the same!
Best,
Frank