On the Expense Report there are two custom buttons: Approve and Reject. Those buttons are created using workflow. How to trigger a client script on that button click?

a bit of a hack and unsuported... i injected the below into a html field on the record via a worklfow - essentially doing what you're chasing. ' <script> var buttonid = "custpageworkflow1639" document.getElementById(buttonid).onclick=function(event){ //somecode NS.Workflow.buttonClick(buttonid); } </script> '