This is from an old Amigopod doc. I haven't tried it myself. Let us know if it helps.
User Detection on Landing Pages
When mac is passed in the redirect URL, we have the ability to detect the user and present a customized message on the landing page.
Navigate to Administrator > Plugin Manager > Manage Plugins: MAC Authentication: Configuration and enable MAC Detect.
Edit the header of your redirect landing page (login or registration) and include the following:
<p>{if $guest_receipt.u.visitor_name}
Welcome back to the show, {$guest_receipt.u.visitor_name|htmlspecialchars}!
{else}
Welcome to the show!
{/if}</p>
For debugging purposes, include the following to see all the fields available:
{dump var=$guest_receipt export=html}