The best option for you is to use the Windows Azure Cache Service (Preview). This will let you share session state among multiple instances of your application.
You will need to sign up for the preview service. the cache core is mature and customers are encouraged to use this service in their production applications.
This service lets you use the cache from multiple cloud services, allowing you to share data across applications too. Also, this removes the dependency for you to use XL VMs just because In-Role Cache wasn't supported on A7.
Also, to be clear, Cache has 3 flavors - 1. Shared Caching - This is a multi-tenant service and is being deprecated 2. In-Role Caching - Here the cache is deployed in your cloud service and can only be used from within the cloud service. This is of two types - Co-Located & Dedicated. 3. Cache Service (preview) - The newly released service.
There is nothing called Dedicated Shared Cache. :)