I have discussed this issue with the developer, this feature has not been supported in the current build of WebGrid 7.

However, the next build of WebGrid 7 will be enchanced to include this feature.

The next build of WebGrid 7 will support this feature which will be released in a few weeks. Here is a snippet of the GetData function:

[WebMethod]
public Object GetData(DataSourceSelectArguments selectArguments)
{
    SqlCommand retrieveComm = new SqlCommand();

    retrieveComm.Connection = new SqlConnection(ConfigurationManager.ConnectionStrings["SqlNorthwindConnectionString"].ConnectionString);

    retrieveComm.CommandText = "SELECT TOP 15 * FROM [Products]";

    SqlDataAdapter da = new SqlDataAdapter();
    da.SelectCommand = retrieveComm;

    DataTable dtTemp = new DataTable();
    da.Fill(dtTemp);

    if (selectArguments.OperationType == SelectOperation.SelectData)
    {
        ISDataTable isTable = DataTableConverter.ConvertFrom(dtTemp);
        return isTable;
    }
    else if (selectArguments.OperationType == SelectOperation.SelectCount)
        return dtTemp.Rows.Count;

    throw new InvalidOperationException("Unsupported operation type!");
}

Here is the snippet of the WebGrid client binding property:

<ClientBindingSettings DataLoadMode="PagedData" DataSourceType="WebService"
    ServiceUrl="CustomService.asmx">
    <ServiceMethods SelectMethod="GetData" />
</ClientBindingSettings>

It is very similar with the snippet code you posted