Conditionally display sitewide notification?

Also, how can I access the content of unread announcements for the current user (to load in a $var in order to be displayed in a custom banner, for example)?

You can use fep_header_notification filter hook to change header notification.
You can use fep_get_user_announcement_count( 'unread' ) function to get unread announcement count.

Ok Great, thanks.

I decided just to use simple html to add a notification in the header above the sidebar to show Message & Announcement counts only when there are any unread, and a scrolling banner across the top of the site when there is an unread announcement. In case anyone else wanted something like this, here’s the code I used…

In header.php:

<!-- Position this code to show the scrolling banner in the desired location of the header -->
<?php 
  unset ($new_msg);
  unset ($new_ann);
  $new_msg = fep_get_new_message_number(); 
  $new_ann = fep_get_new_announcement_number();
?>				
<?php if ($new_ann != 0): 
  if ($new_ann == 1){
    $s="There is an important new site announcement";
  } else {
    $s="There are " . $new_ann . " important new site announcements";
  }	
  $s = $s . " - please click here to read now...";
  ?>
  <a itemprop="url" href="<?php echo esc_url( home_url() . "/private-messages/?fepaction=announcements"); ?>" class="announce">
  <marquee behavior="scroll" direction="left"><b><?php echo $s; ?></b></marquee>
  </a>
<?php endif; ?>	

<!-- Position code to show # of unread messages/announcements in the desired location -->
<div style="text-align:right">
  <?php if ($new_msg + $new_ann != 0): ?>
    <a itemprop="url" href="<?php echo esc_url( home_url() . "/private-messages/?fepaction=messagebox"); ?>" class="notice">
    <b>Unread Messages: </a><?php echo $new_msg; ?></b>
    <?php if ($new_ann != 0): ?>
      &nbsp;&nbsp;<a itemprop="url" href="<?php echo esc_url( home_url() . "/private-messages/?fepaction=announcements"); ?>"class="notice">
      <b>TEST_Site Announcements: </a><?php echo $new_ann; ?></b>
    <?php endif; ?>
  <?php endif; ?>
</div>

Added classes to Style.css:

a.notice {
  color: red;
}
a.notice:hover {
  color: #ff6666;
  text-decoration: none;
}
a.announce {
  color: blue;
}
a.announce:hover {
  color: #8383af;
}

you can change <?php echo esc_url( home_url() . "/private-messages/?fepaction=announcements"); ?> to <?php echo fep_query_url( "announcements" ); ?> so that you can use any page as PM page