Here is how I usually do it: just add these members to the MyConfig class:

    public class MyConfig : ConfigurationSection
    {
        private static MyConfig _current;
        public static MyConfig Current
        {
            get
            {
                if (_current == null)
                {
                    switch(ConfigurationStorageType) // where do you want read config from?
                    {
                        case ConfigFile: // from .config file
                            _current = ConfigurationManager.GetSection("MySectionName") as MyConfig;
                            break;

                        case ConfigDb: // from database
                        default:
                            using (Stream stream = GetMyStreamFromDb())
                            {
                                using (XmlTextReader reader = new XmlTextReader(stream))
                                {
                                    _current = Get(reader);
                                }
                            }
                            break;


                    }
                }
                return _current;
            }
        }

        public static MyConfig Get(XmlReader reader)
        {
            if (reader == null)
                throw new ArgumentNullException("reader");

            MyConfig section = new MyConfig();
            section.DeserializeSection(reader);
            return section;
        }
    }

This way, you have nothing to change in the MyConfig class, but you still need to change the way your customers access it with this kind of code:

string myProp = MyConfig.Current.MyProperty;