Can I add banners to any custom positions?

Yes, in addition to adding promo banners to the places like above cart, sidebars, product/category page, etc. there is also a possibility to use the banner block in any position. The code is {{block type="ambanners/container" name="ambanners.right" template="ambanners/container.phtml" position="2"}}.

Note that this code is applicable to CMS pages only; other types of pages will require some template customizations.