To do a projection, you would do something like

Query q = pm.newQuery("SELECT myField FROM mydomain.MyClass WHERE featured == true");
List<String> results = (List<String>)q.execute();

where String is the type of my field. Any basic JDO documentation would define that. Internally GAE/J will retrieve the Entity, and then in the post-processing before returning it to the user it is manipulated into the projection you require.

As Nick pointed out in the other reply, this gives no performance gain over doing it yourself ... but then the whole point of a standard persistence API is to shield you from such datastore-specifics of having to do such extraction; it's all provided out of the box.