Hook into the rendering of a WordPress Template

You can, but WP doesn’t offer you the possibility to do it in a nice way, so you have to use output buffering.

Something like:

// before template is included
add_action('template_redirect', function(){

  // you can do some checks here
  // perhaps you need this only a certain page?
  if(!is_home())
    return;

  // capture output from here on...
  ob_start(function($html){

    // this function runs after the capture ends,
    // you can replace your tags here 
    $html = strtr($html, array(
      '<render:jsp>'  => '<!--',
      '</render:jsp>' => '-->',
    ));

    return $html;

  });

});

The contents of the buffer are automatically sent by PHP to the output just before the script finishes execution.