Insert *whatever* every nth row in Views
In under Theme: Information you'll find Style output: views-view-table.tpl.php .... Override that. I selected the Theming info as opposed to other template overrides available because that is the one that has a foreach for rows. In your override you can add a counter to the foreach() along with an if($counter%10 == 0) { ... } with some markup or call to a block or something like that.