Adding Inline Content (Example here under HTML Content) with the WS-Nivo System Gallery Plugin for Joomla is easy. But please keep a few things in mind:
For your Inline HTML you have to place a DIV Container into the Content of the Page, for example like this:
<div style="display: none;" id="hidden-content">
<h2>Inline HMTL</h2>
<p>Vivamus sagittis lacus vel augue laoreet rutrum.</p>
</div>
And then simply "call" the Lightbox with this command:
<a href="#hidden-content" data-lightbox="nivo">Show HTML</a>