Single signon and central dashboard for multisite subsites

I have been scouring the Internet to find a solution that I can possibly expand from but I’m simply coming up dry. So I ask the community for help with this.

Basically I have a large multisite ecommerce installation using Marketpress with several stores (subsites) and I am trying to come up with a way for users to be able to have two or more subsites while using the same login (single-signon). The concept that I am thinking of is that a user logs into WordPress, arrives at a central dashboard which shows stat analysis for sales / visits and total number of products listed per store and views per product. The subsite admin will be able to click on a store link to then be redirected to the subsite dashboard and individually manage the store.

I have inquired about this directly to WPMU Devs but I have not received any helpful feedback on this matter. Does anyone know of any plugins that may be of use or the general capability of multiple subsite management. I have already checked out managewp and infinitewp however they will not necessarily work because I am managing all subsites with clones of the main theme and plugins.

Any help or ideas are greatly appreciated.