This will allow a user of Qt Quick Controls to specify a style:

ApplicationWindow {
    onBlah: {
         Qt.application.name = "Blah";
         Qt.application.style = "MyStyle";
    }
}

Another alternative which has previously been mentioned is to use a .json file. This file would specify things that an end user shouldn't have access to, such as the items above, and could be embedded into the executable. Plugins like Qt Quick Controls could then check the file's existence, and do nothing if it doesn't exist. This file could be named the same as the application executable. This approach seems more "toolable" from the perspective of Qt Creator.