Figured it out, the change is very simple.
In toc.php, find the function shortcode_sitemap_posts.
Modify the first line to look like this (add the postcat portion):
extract( shortcode_atts( array(
'order' => 'ASC',
'orderby' => 'title',
'separate' => true,
'postcat' => $this->options['postcat']
), $atts )
);
Modify the second line to look like this (add the category_name portion):
$articles = new WP_Query(array(
'post_type' => 'post',
'post_status' => 'publish',
'order' => $order,
'orderby' => $orderby,
'posts_per_page' => -1,
'category_name' => $postcat
));
Sample useage is like this:
[sitemap_posts postcat="SOPs" no_label="true" separate="false"]