Even if you derive from QComboBox, you will still have to overwrite a lot of things and implement them your own, at least painting and event handling.

Since you would have to "work around" assumptions in the QComboBox code this is probably more work then to implement your custom combobox yourself.
Especially since you might know restrictions that can make certain things unnecessary, e.g. not needing to handle text input if your comboboxes aren't editable.

Cheers,
_