Dropdown list for input parameter

I am using Toad for Oracle Expert Edition I would like to execute a procedure from the schema browser and have a drop-down list of possible choices for an input parameter. I am hoping there is some way to cause this behavior from the procedure itself. A list of dynamic values from a query would be great but a list of literal values would do in a pinch. Any help would be much appreciated as I have searched high and low without success.