The Admin SDK APIs function with Two Legged OAuth just fine, I'm using the Directory API in my marketplace app today. You'll need to go into your Vendor Profile page on the marketplace, click "Register for additional APIs" and enable Admin SDK. You'll also need to add the needed API scopes to your manifest.
Also note that the google-api-python-client at least does not support OAuth 1.0 any longer so you'll need to either perform the authentication manually or utilize the old GData library to get the correct auth headers.