opera.addEventListener('BeforeScript'
  ,function(e) {
    if (e.element.src && /\/arbitrary-script\.js$/i.test(e.element.src)) {
      e.element.setAttribute('src', '/scripts/my.js');
      opera.postError('replaced script');
      opera.removeEventListener('BeforeScript', arguments.callee, false);
    }
  }
  ,false
);