Martin, that is an interesting viewpoint, and one I considered, but didn't state, because it seems much too restrictive.  Most CGI programs are written in scripting languages, not compiled to .exe.  So it seems the solution should allow for launching at least Perl and Python scripts, as well as .exe.  Whether subprocess.Popen can directly execute it, or whether it needs help from the registry or a #! line to get the execution going is just a matter of tweaking the coding for what gets passed to subprocess.Popen.  Declaring the definition based on what the existing code can already do is self-limiting.

Another possible interpretation of executable might be PATHEXT environment variable, but that is similar to declaring a list in the server source, which I did mention.  One might augment the other.