I have accomplished this by writing a little php script that cleans up the link, and then does a redirect to your server. So your registry setting points to wherever you host the php script with the ?command=%1 and then the following code will clean up what is passed in "command"..
Code:
<?php
$mail_domain="http://mail.domain.com";
$mail_cmd="?app=mail&view=compose";
$NEW=split (":",$_GET['command']);
$cmd="to=".$NEW[1];
$cmd=str_replace("?","&",$cmd);
$url=$mail_domain.'/'.$mail_cmd.'&'.$cmd;
header ("Location: $url");
?>