The top example in your image would be harder to implement as it requires being able to set a different color for each item.
The bottom example can be implemented by just adding a default text. When the port becomes unavailable you just have to deselect the item, remove it from the list and set the default text.
So if it is enough for you then I'll just add a DefaultText property to ComboBox and a DefaultTextColor property to ComboBoxRenderer.
Edit: the default text was added to ComboBox (in the version that can be downloaded on
github)