EF6 (Nightly/RC1) - Mapping collection via same column to multiple classes using TPH

Question

I'm trying to use table per hierarchy (TPH) with two classes inheriting from an abstract base:

public abstract class StatusLog : EntityBase
{
    public StatusLog()
        : base()
    {
        StatusChangeValidations = new List<StatusChangeValidation>();
    }

    public virtual List<StatusChangeValidation> StatusChangeValidations { get; set; }
    public int StatusId { get; set; }

}

public class DisputeStatusLog : StatusLog
{
    public virtual Dispute Dispute { get; set; }
}

public class StatementStatusLog : StatusLog
{
    public virtual Statement Statement { get; set; }
}

I've mapped them like this:

public class StatusLogMap : EntityTypeConfiguration<StatusLog>
{
    public StatusLogMap()
    {
        // Primary Key
        this.HasKey(t => t.Id);

                    ...
    }
}

public class DisputeStatusLogMap : EntityTypeConfiguration<DisputeStatusLog>
{
    public DisputeStatusLogMap()
    {
        // Relationships
        this.HasRequired(t => t.Dispute)
            .WithMany(t => t.StatusLogs)
            .Map(m => m.MapKey("ObjectInstanceID"));
    }
}

public class StatementStatusLogMap : EntityTypeConfiguration<StatementStatusLog>
{
    public StatementStatusLogMap()
    {
        // Relationships
        this.HasRequired(t => t.Statement)
            .WithMany(t => t.StatusLogs)
            .Map(m => m.MapKey("ObjectInstanceID"));
    }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
{
        modelBuilder.Entity<StatusLog>()
                    .Map<DisputeStatusLog>(e => e.Requires("ObjectID").HasValue("28"))
                    .Map<StatementStatusLog>(e => e.Requires("ObjectID").HasValue("111"));
}

I'd expected this to work, but it causes a validation exception immediately:

ObjectInstanceID: Name: Each property name in a type must be unique. Property name 'ObjectInstanceID' is already defined.

Adding a foreign key property for ObjectInstanceID and changing the mappings to use HasForeignKey instead of MapKey results in a slightly different error:

ObjectInstanceID: : There is no property with name 'ObjectInstanceID' defined in the type referred to by Role 'StatusLog'.

Is there something wrong with my mappings, or is this a scenario that Entity Framework doesn't support yet? I'm currently using the 9/23 nightly build of EF 6.01 as I'd read on the EF CodePlex site that it may have fixed this problem, but I had the exact same error with EF 6 RC1.

I know I can fix this by splitting these classes up into multiple tables and using TPT, but would really prefer to avoid this as it's an existing database.