Add a new web login page with a blank skin. Check Provide a custom login form.
Remove all text from Header and Footer and add the following, replacing the appropriate values:
{if $smarty.server.SSL_TLS_SNI == 'devicereg.millsaps.edu'}
<meta http-equiv="refresh" content="0;url=https://clearpass.millsaps.edu/guest/mac_create.php">
{else}
<meta http-equiv="refresh" content="0;url=http://www.millsaps.edu">
{/if}
Then in Policy Manager, set that web login to be the landing page (Administration > ClearPass Portal)