frame.html

<b>iframe</b>

page.html

<title>My page</title>

<iframe src="frame.html"></iframe>

<script>
  setTimeout(function() {
    document.getElementsByTagName('iframe')[0]
      .setAttribute('name', 'myFrame');
    console.log('set');
  }, 1000);
</script>

test.js

;(function() {
    'use strict';
 
    casper.start('page.html', function() {
        this.test.assertTitle('My page');
        this.wait(3000);
    });

    casper.then(function() {
        // this will fail when 'name' attribute is added using js
        casper.withFrame('myFrame', function() {
            this.debugPage();
        });
    });
 
    casper.run(function() {
        this.test.done();
    });
})();