Based on my test, using query string select parameter I could successfully filtered the data in the Grid. Attached is the page I used to test the scenario.

The GenericObject class is already available on the WebGrid provided sample under the folder App_Code/GenericObject. Plese add a new function GetCustomersFiltered, here is the snippet:

// using Generic Collection
[System.ComponentModel.DataObjectMethodAttribute(System.ComponentModel.DataObjectMethodType.Select, true)]
public List<Customer> GetCustomersFiltered(string ContactTitle)
{
    List<Customer> customers = new List<Customer>();

    try
    {
        this.Connection.Open();

        string cmdText = "SELECT " +
            "CustomerID, CompanyName, ContactName, ContactTitle, Address, City, Region, " +
            "PostalCode, Country, Phone, Fax FROM Customers";

        if (!string.IsNullOrEmpty(ContactTitle))
            cmdText += " WHERE ContactTitle = '" + ContactTitle + "'";

        OleDbCommand command = new OleDbCommand(cmdText, this.Connection);

        OleDbDataReader reader = command.ExecuteReader();

        while (reader.Read())
        {
            Customer customer = new Customer();
            customer.CustomerID = reader.GetString(0);
            customer.CompanyName = reader.GetString(1);
            customer.ContactName = reader.GetString(2);
            customer.ContactTitle = reader.GetString(3);
            customer.Address = reader.GetString(4);
            customer.City = reader.GetString(5);
            customer.Region = reader.IsDBNull(6) ? "" : reader.GetString(6);
            customer.PostalCode = reader.IsDBNull(7) ? "" : reader.GetString(7);
            customer.Country = reader.IsDBNull(8) ? "" : reader.GetString(8);
            customer.Phone = reader.IsDBNull(9) ? "" : reader.GetString(9);
            customer.Fax = reader.IsDBNull(10) ? "" : reader.GetString(10);

            customers.Add(customer);
        }

        return customers;
    }
    finally
    {
        this.Connection.Close();
    }
}

Without query string the page will list all customer, if we wish to show all the Owner, the query string will be ?ContactTitle=Owner