The theme I've been using has the line:
<meta name="robots" content="<{$xoops_meta_robots}>" />

..And I'm using the latest piCal index.php which attempts to assign the "nofollow" directive:
$xoopsTpl->assign( "xoops_meta_robots" , $meta_robots ) ;

But for some reason the page is compiled with the Robots rule as set by the main Xoops preferences. For now I'm using "Disallow" but would rather have "nofollow" so more of the calendar gets indexed.