You can't add constraints to object types at all - constraints are on a table. If the object is not instantiable, you can't have a table based on it, ergo no constraints on it.
I guess the answer would be to make the object type Instantiable. Then you can create a table based on it with constraints. Of course, these constraints won't be "inherited" by the child types.
All that said, however, I personally avoid Oracle object types when I can.