It sounds to me that you need something that is installed at the template level. You can create a scrollable div which is accessed by a link or button, create a page containing your simplenavi markup, and insert the navigation into the div with p_wiki_xhtml:
<div id="name_of_div" style="overflow:scroll">
<?php echo p_wiki_xhtml ("page_with_simplenav"); ?>
</div>
This could also be implemented as a pop-up.