Well, on some website I once read that because most operating systems use "/" instead of "\", Windows supports "/", too. You can try it, by e.g. acessing "C:/Windows" in the Windows explorer, it will just correct it to the "\" notation.
The website recommended always using "/", because that way it's easier to port code from Windows to other operating systems. Even though Gamestudio only works on Windows, I like to use it there, too, because it seems much cleaner than having to use "\\".

I tried using ent_create with a subfolder and "/" and it worked for me.

Maybe you could just use PRAGMA_PATH or a WDL with a path statement?