So, I've written a solution, not sure if this is the 'write' way to do it, but it seemed the easiest in my eyes.
The problem I had is that because Q2A is effectively created on the fly, there were no specific files I could set in IIS to allow for anonymous access, so I added this to the bottom of the file /qa-include/qa-feed.php
$rawrss = implode("\n", $lines);
$filename = 'rss.xml';
$rsscontent = $rawrss;
if (is_writable($filename)) {
if (!$handle = fopen($filename, 'a')) {
exit;
}
if (fwrite($handle, $rsscontent) === FALSE) {
exit;
}
//done
fclose($handle);
} else {
//error check file is writeable
}
This uses the already defined $lines variable and writes this to a static RSS file called rss.xml in the root. This means that everytime the RSS feed is viewed (I only need one RSS feed, I guess you can do some clever matching to write different files), the file is updated. I also setup a scheduled task in PHP to run this file every 2 hours (just in case no one views it).
Then in IIS I simply set this file and this file only to have anonymous access, and bam, I can now use the feed in SharePoint, Blackboard and other services.