I haven't tried it at all, but in includes/cron/rssposter.php, around line 317, there's this:
Code:
$pagetext = $feed['xml']->parse_template($body_template, $item);
if ($feed['rssoptions'] & $vbulletin->bf_misc_feedoptions['html2bbcode'])
{
So if you wanted to do something to the text before it's posted you could insert it between those first two lines. Also there's a php function html_entity_decode() which you might be able to use to convert any of those kind of things without listing them all in a str_replace().