One solution would be to create your dropdown with a second repeater. This would allow you to avoid cloning the out-of-the-box one. With this option you would:
Create a dropdown using a repeater, bound to your SQL query, and using customer transformations to render the dropdown.
Add script so that when the dropdown is changed a value is added to your URL as a query string.
Add a repeater to list the results on the page. Use a macro in the WHERE property to check the query string value and create the where condition.