Sharepoint - JavaScript: How to reuse the "Working on it..." pop in

Juergen,

The below article should help:
http://www.collabware.com/blog/2013/03/22/tips-tricks-sharepoint-2013-modal-dialogs

You have SP.UI.ModalDialog.showWaitScreenWithNoClose which can be used..

function RequestEnded(sender, args) {
  try {
    waitDialog.close();
    waitDialog = null;
  } catch (ex) { }
};

function RequestStarted(sender, args) {
   ExecuteOrDelayUntilScriptLoaded(ShowWaitDialog, "sp.js");
};

function ShowWaitDialog() {
   try {
      if (waitDialog == null) {
         waitDialog = SP.UI.ModalDialog.showWaitScreenWithNoClose('Processing...', 'Please wait while request is in progress...', 76, 330);
      }
   } catch (ex) { }
};