Ah, so what you're saying is that this is a problem when creating a bdist_wininst, *while on Windows*.  Got it.  Thanks!

The problem is that the shebang of gui_scripts is not changed to #!pythonw.exe!

In my case new_header[2:-1] is "python.exe" (as set in get_script_header line 1420), thus os.path.exists(new_header[2:-1]) is false, and sys.platform!='win32' is false as well. hdr is never changed, and the gui script is started showing an ugly console

So, under what input conditions does this produce a problem?  It seems like it would have to be one where the pythonw.exe doesn't exist, and one is installing on Windows.

When creating a wininst, get_script_header alwasy sets the executabe to "python.exe".

Thus, in get_script_args() the code to check if the header should actually be changed should be updated to:
if os.path.exists(new_header[2:-1]) or wininst or sys.platform!='win32':
    hdr = new_header
else:
    hdr = header