James, what you offer as solution is not the scenario I need. The namespaces app.bill needs to be observable by itself. It could be a string or an object, doesn't really matter. My use-case requires that when "app.bill" (the var, not the content!) changes,
listeners of that object "app.bill" are triggered.
So "app.bill.amount = 1" is NOT my scenario. That works already. But I have UI elements that should not be shown when "app.bill" is null. So the UI elements bind to this property exactly like this:
<span data-win-bind="style.display:app.bill binding.displayIf">
<span data-win-bind="textContent:app.bill.amount"></span>
</span>
My use-case is "app.bill = new Bill()" or "app.bill = null"