Hi Rik
As Mihnea is showing if you use an HTML offer and add a script to handle the re-direct you can preserve your non-URL-encoded colons. Here is a sample offer that will simply change the subdomain from 'www' to 'www2' in the url for the redirect. Hence, everything in the path and query portion will be maintained as is.
<script type="text/javascript"> var oldURL = document.location+""; var newURL = oldURL.replace("www","www2") window.location.replace(newURL); </script>