Hi Tom
Currently there's no way to fake fields in the AAA API
The alternatives are:
1. Wrap the field with property and use only the property when accessing the field. I think this is the best choice putting the testing issue aside, If you'll want to add some logic in the future when accessing the field you'll have to change the code only in one place.
2. Use
ObjectState
The ObjectState class will let you change values of fields.
ObjectState state = new ObjectState(typeof(IncHolder));
state.SetField("_Parent", 5);