Is it acceptable to modify Drupal's composer.json file?
It is possible to consume Drupal 8 into your own composer.json file:

{
    "name": "davidbarratt/mydrupalproject",
    "require": {
        "drupal/drupal": "8.*"
        "composer/installers": "1.*"
    },
    "repositories": [
        {
            "type": "package",
            "package": {
                "name": "drupal/drupal",
                "type": "drupal-core",
                "version": "8.0.0-dev",
                "source": {
                    "url": "http://git.drupal.org/project/drupal.git",
                    "type": "git",
                    "reference": "8.x"
                }
            }
        }
    ],
    "minimum-stability": "dev"
}


There's a pull request to composer/installerswill allow installation of drupal-core project types:
http://github.com/composer/installers/pull/38

Would love your input on the above pull request, as well as this other one in the queue that allows installation of modules/themes/profiles.

Another solution that currently works is the Composer Manager module, which facilitates composer.json use for modules. It's currently available for Drupal 7, and will be available for Drupal 8 once the Drupal 8 alpha releases are close.

Thanks a lot,
Rob