Unfortunately the _ModerationStatus field is not show in the form and therefore you cannot access it with getFieldValue, but must use a query to get the value. Here is and example you can use in your custom js:
var item = spjs.utility.getItemByID({
"listName": _spPageContextInfo.pageListId,
"id": spjs.dffs.data.thisItemID,
"viewFields": ["ID", "_ModerationStatus"]
});
alert(item._ModerationStatus);
Let me know how this works out.
Alexander