If I attribute it directly or add it to the helper class it does not work.

Is this possible?

 

It looks like we don't have this documented, but you can "extend" the PropertyMetadata partial class for the entity to add the static DataEntityProperty field.  Something like this:

 

public partial class Customer {

  public string ACustomProp { get; set; }
 
  public partial class PropertyMetadata {

    public static readonly DataEntityProperty<Customer, string> ACustomProp  =
       new DataEntityProperty<Customer, string>(propertyName: "ACustomProp",
                                      isNullable: false,
                                      isPartOfKey: false,
                                      concurrencyStrategy: ConcurrencyStrategy.None,
                                      isAutoIncrementing: false,
                                      relatedNavigationPropertyName: null,
                                      isNativeProperty: false);
  }
}

 

DevForce will discover the property definitions when the model is accessed.  You can then obtain property metadata from the EntityMetdataStore.