Every doc I can find discusses editing applicationHost.config in notepad to enable this.
The real need:
a) Via appcmd.exe or cmd line tool, enable dynamic compression to be configured in web.config (e.g. to set applicationHost.config -> "httpCompression" -> overrideModeDefault= to "Allow"
b) If 'a' cannot be done, then how, via appcmd.exe or cmd line tool, to enable site wide gzip dynamic content compression for mime type application/x-javascript
I just spent half an hour exploring appcmd.exe, and searching the net, and have seen no examples of using appcmd.exe to flip the overrideModeDefault values
Here is the way to unlock it: