Hi, after adding your plugin I realized my RSS feed was broken. I narrowed it down to rewrite_content where the DOMDocument is treated at HTML. My non-php-non-wordpress-programmer hack was to search for '<?xml' in the $content, then use that to decide between loadHTML/loadXML and saveHTML/saveXML.
Seems like a poor workaround, I'm sure you can solve it more elegantly.
Thanks