the iOS webkit (webcontent viewer) has many disabilities compared to mobile safari (they are two very different things) I haven't used the orientation property myself but if it requires you to request the method through a "node" it will require hard coding to work in the webkit. Its worth taking it to an apple developer forum to see what is actually supported to call through the webviewer.
It's a problem on the way to awesome interactivity.
For example our gravity scenario (with G-Sensor data) does not works correctly, becouse we can't determine current orientation.
Property window.orientation in undefined in DPS WebView and deviceMotion return angle with no orientation correction.
Adobe please create custom JS property and event for orientation (ask your PhoneGap guys to help your =) )