Yes, it would be different for MODx Evolution. I also notice that I left out the last few lines when I posted it (fixed above).
For Evolution:
replace the last lines:
$output =& $modx->resource->_output;
$output = preg_replace_callback("#(<fixedpre>)(.*?)(</fixedpre>)#s",
"quote_meta", $output);
with this
$modx->documentObject['content']=
preg_replace_callback("#(<fixedpre>)(.*?)(</fixedpre>)#s",
"quote_meta", $modx->documentObject['content']);
For Evolution, I think you might also have to connect the plugin to OnLoadWebDocument instead of OnParseDocument, but try OnParseDocument first and let me know if it works.
