Thank-you. I was able to get it to work with this:
var slidingZone = $find("<%= RadSlidingZone1.ClientID %>");
var slidingPane = slidingZone.getPaneById('RadSlidingPane1');
var l = document.getElementById(label);
var title = ('Contacts for ' + l.innerHTML);
However I was reading the documentation for the Prometheus Sliding Pane members and events and it says that eventArgs contains the pane being passed in as paneObj and you can use eventArgs.paneObj.ID to retrieve the pane ID that is calling the function. I could not get this to work as I get an error that paneObj.ID is null or not an object. It would be convenient for me to know which sliding pane is calling the function as I have several panes calling it and they all set the same title property.