Currently, there is no way to get all the subscription identifiers associated with your Windows Azure Live Id from the Windows Azure REST APIs. All API calls require that you know your subscription identifier before making the call.
There is also no way to make a Windows Azure REST API call using your Live Id (like the Windows Azure Management Portal does) you need to uploaded a Management Certificate using the Windows Azure Management portal and pin it to a subscription before you can start making REST API calls.
Interesting to note, it appears that the Management portal supplies its own Management certificate to Windows Azure when you request a new subscription. If you delete that Management Certificate my guess is that you will no longer be able to manage your subscription via the Management Portal.