No sorry, there’s no way I can think of to easily achieve this programtically (with my limited PHP knowledge at least). Scribu, the original author/maintainer of the plugin, may be able to suggest a solution if you ask over on the main plugin page.

I had a play around however and you can achieve what you’re after with a little jQuery/CSS magic! (I’m not suggesting that this is by any means the best way to do it and is is certainly not as elegant/clean as doing it in the php).

In your list format field on the Fancy Archives option screen add the %excerpt% place holder wrapped in a span and give it a unique class name:

<span class="archive_excerpt">%excerpt%<span>%post_link% in %category% - %comment_count% comments

Then in your theme’s stylesheet add:

span.archive_excerpt { display: none; margin-bottom: 5px; }

and finally in the theme’s header.php file add the following script:

<script type="text/javascript">
	$(document).ready(function($){
		$('.pane ul li:first-child span.archive_excerpt').css("display", "block");
	});
</script>

And you’re done!

I did note while testing this though that there seems to be a bug in the way the excerpt is added to the archive. If a post doesn’t have an excerpt it displays the excerpt for the archive page itself.

I’ll raise this issue with Scribu so it will hopefully be fixed in a future release of the plugin.

Hope this helps!