The JSON editor allows the designer to click a + sign in the Perspective property list and it adds a new entire array structure of properties complete with default values. (alternatively, right-click : Structure, Insert, Default) It'd be nice if a script function were provided to do the same, while allowing overwriting specified entries. e.g. sys.pers.addElement("myMap.props.layers.ui.marker",overrides= {"color":"#4190F7","lat":41.7,"lng":-87.6})

In the following forum post Paul says it's not currently available.