dynamic connection string for injected repository

I'm building a multi tenant (db per tenant) application.

• the application - .net MVC4
• the data layer entity framework code first repositories
• the repositories are injected into the application via structure map

The structuremap config:

var connectionString = ConfigurationManager.ConnectionStrings["AccessControlDB"].ToString();ObjectFactory.Initialize(x =>    {        x.For<IAccessControlContext>().Use<AccessControlContext>().Ctor<string>("connectionString").Is(connectionString);        x.For<IGenericRepository<Identities>>().Use<GenericRepository<IAccessControlContext, Identities>>();        x.Scan(scan =>        {            scan.AssembliesFromApplicationBaseDirectory();            scan.ExcludeNamespace("StructureMap");            scan.WithDefaultConventions();        });    });

A new requirement for having each tenant isolated by database has come up, so having tenantIDs is no longer sufficient.

I've gotten as far as having a meta database to store the data required to construct the connection strings, but I'm at a loss on how to pass the connection string to the repository.

At first, I thought I could just expose the connection string as a property, but there is no way to change the connection string after the context has already been instantiated by structuremap.