I would like to know if it is possible - and how - to define a list a Google Apps users as a list of co-administrators of a Windows Azure subscription? Basically, the process would be equivalent to replacing the Microsoft Live authentication by the Google authentication.
I do not believe that this is possible.
At the moment only Microsoft accounts or users in the subscription's default Windows Azure AD directory can be configured as co-administrators.
I believe The link posted by @ceejayoz talks about configured SSO to Google Apps from WaaD, which is more about using WaaD users to access Google Apps