You could call the disableCombo function, passing the correct ComboBox ClientID whenever you find appropriate (or is needed).
For instance, if certain conditions at the server-side are met, you could emit a script, which calls the disableCombo function.
Regarding the ReadOnly functionality you are referring to, it would require major modifications of the existing code and changing many internal aspects of RadComboBox. This could break existing implementations using the control, so it is more likely that we preserve the current behavior.
the Telerik team