There's no ```env``` subdirectory on Windows (and no standard sysconfig location like bin_dir or lib_dir)  I put it in sys.prefix because that's where the standard Python venv puts its parameter file (although the reason for that file is different).

If it should go in <venv>/etc on Unix I could do that - but I wouldn't want to for Windows. Also, should virtualenv create an etc directory? Or should the user be required to create the directory in order to add the per-venv file (and know that they need to do that on Unix but not Windows)?

Personally I think it's more compexity than the feature warrants, but I can do it if the Unix people think it's worth it.