First of all, great plugin!
I installed this plugin to make a site work when it has a different sub-domain when viewing the site over ssl. It almost worked but there were some problems because some links used http instead of https.
I was able to fix this problem by adding:
$parts['scheme'] = ($_SERVER['HTTPS'] == "on")?"https":"http";
$parts = parse_url($url);
In the "filter_url" function.
Hope that helps and thanks for the great plugin!