I was thinking about an simple php redirect script and send the server you want to connect to via a HTTP Get variable.
For example:
http://example.com/?s=127.0.0.1
However I had some difficulties with the dots in the IP (I dont know if you can support dots in a GET variable trough the htaccess file)
But anyways, redirecting to a steam:// url from php works!
Edit:
PHP file should be something like this:
<?php
$server = $_GET["s"];
header('Location: steam://connect/{$server}/');
exit();
?>
Sorry for my broken English, I hope it made sense to you.