MinGW 5.x installer is actually packaged with the NSIS installer, so it should react to the command input /S for silent install. However like some of the OpenSource community, the Installer fails to respond properly.
If you were to use /S, the installer would simply 'download' the packages and quit.
It seemed the only solution was to repackage the software into an MSI file for distribution. But, repackage is not successful, according to the MinGW page, the only method would be manual Install.