You can change attributes of a given class at runtime without changing that class. So you could write a custom TypeConverter and set it to your classes, something like this:

    TypeDescriptor.AddAttributes(typeof(ComposedType), new TypeConverterAttribute(typeof(FieldsExpandableObjectConverter)));
    TypeDescriptor.AddAttributes(typeof(BaseType), new TypeConverterAttribute(typeof(FieldsExpandableObjectConverter)));

With the following TypeConverter (re-using your FieldDescriptor class):

public class FieldsExpandableObjectConverter : ExpandableObjectConverter
{
    public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, Attribute[] attributes)
    {
        List<PropertyDescriptor> properties = new List<PropertyDescriptor>(base.GetProperties(context, value, attributes).OfType<PropertyDescriptor>());
        if (value != null)
        {
            foreach (FieldInfo field in value.GetType().GetFields())
            {
                FieldPropertyDescriptor fieldDesc = new FieldPropertyDescriptor(field);
                {
                    properties.Add(fieldDesc);
                }
            }
        }
        return new PropertyDescriptorCollection(properties.ToArray());
    }
}