The behaviour that you describe can be easily achieved with e few lines of code.
That is why there is no need to implement a feature in RadComboBox for it.
I can suggest you inherit the RadComboBox class and implement there a Mode property(with two values: "normal" and "label") and a Label. In this way you will be able to set the to set the RadComboBox(or the Label) Visible property
the Telerik team