I can confirm this. It works on 'OnParseDocument' but not on 'OnWebPagePrerender'. Actually, the $modx->documentOutput was simply empty on OnWebPagePrerender as I remember.
<em>The Earth is not the hell, and people aren't cursed, and joy in not a sin, but a gift of God.</em> (Lesia Ukrainka)
$output =& $modx->resource->_output