It is easy to do if you understand a bit about how WordPress works.
The basic answer is that the message is sent through a filter which can be used to add whatever text you want. It is called by the ShowPosts plugin like this:
echo apply_filters( 'wvr_show_posts_no_posts', 'No posts found.');
So you need to add a filter to your theme that returns the wording you need. The standard way to do this is with a child theme.
You would add code like this to the child functions.php file:
return 'Watch this space!';
If you have Weaver Xtreme Plus, you can add that code to the Actions and Filters section of the Site <HEAD> Section option.