Here's the composer file used in my demo during my talk at WordCamp Norrkoping

composer.json

{
    "name": "edmund.turbin/wordpress-starter",
    "description": "This is a basic wordpress install with some changes to the wp-content and core file locations",
    "type": "dev",
    "license": "GPL",
    "authors": [
        {
            "name": "Edmund Turbin",
            "email": "edmund.turbin@wpengine.com"
        }
    ],

    "repositories":[
        {
            "type": "composer",
            "url": "http://wpackagist.org"
        },
        {
            "type": "package",
                "package": {
                        "name": "roots/sage",
                        "type": "wordpress-theme",
                        "version": "dev-master",
                        "source": {
                            "url":"https://github.com/roots/sage.git",
                            "type": "git",
                            "reference": "master"
                        }
                }
        }

    ],

    "minimum-stability": "dev",

    "require": {
        "composer/installers": "~1.0",
        "wpackagist-plugin/debug-bar":"*",
        "wpackagist-plugin/debug-objects":"*",
        "johnpbloch/wordpress": "*",
        "roots/sage": "*"
    },

    "extra": {
        "wordpress-install-dir": "wp",
        "installer-paths": {
            "content/plugins/{$name}": ["type:wordpress-plugin"],
            "content/themes/{$name}": ["type:wordpress-theme"],
            "content/mu-plugins/{$name}": ["type:wordpress-muplugin"]
        }
    },

    "scripts": {
        "post-install-cmd": [
            "cp wp/index.php ./index.php",
            "cp wp/wp-config-sample.php wp/wp-config.php",
            "sed -i '' \"s/\\/wp-blog-header/\\/wp\\/wp-blog-header/g\" index.php",
            "sed -i '' '2i\\'$'\\n''/** Define variables for custom content directory */'$'\\n' wp/wp-config.php",
            "sed -i '' '3i\\'$'\\n''define ('WP_CONTENT_FOLDERNAME', 'content');'$'\\n' wp/wp-config.php",
            "sed -i '' '4i\\'$'\\n''define ('WP_CONTENT_DIR', ABSPATH . WP_CONTENT_FOLDERNAME);'$'\\n' wp/wp-config.php"
        ]
    }
}