Daniel,
Sure to make your FeatureLayer not appear in the LayerList widget just use this code (line 5):
var statesLayer = new FeatureLayer("https://sampleserver1.arcgisonline.com/ArcGIS/rest/services/Demographics/ESRI_Census_USA/MapServer/5", {
mode: FeatureLayer.MODE_SELECTION,
outFields: ["ObjectID", "STATE_NAME", "POP2000"]
});
lang.setObject('_wabProperties.isTemporaryLayer', true, statesLayer);