You could create a custom s-control that has the values to be selected, and on the selection a defined (by selection) message can pop up. If Ok is clicked you can then write that value to the object just like it would have done natively. That would just require a little HTML s-control with a javascript function to read the selected value, call alert() with the right message and then save (if desired) (ajax toolkit).
Or perhaps I have just engineered the better mousetrap and something like this can be done with a field update to have a field hold the value desired and until that field is blank, a validation rule does not permit saving.
Or perhaps I have just engineered the better mousetrap and something like this can be done with a field update to have a field hold the value desired and until that field is blank, a validation rule does not permit saving.
Bryan