That won't work since the article comments stream needs information from the article to render properly. If you had CB Auto Actions you'd be able to act on the same event CB Activity is to insert its comments stream to insert custom HTML in the same location, but I don't think Joomla has a module position for inserting inside the article.
Is inserting text there exactly necessary or do you just need some further visibility improvements. Like making the comments appear larger? Article comment UX is pretty much the same across all site (at the bottom) so I don't think your users should have a hard time finding this. Making it bigger however should be relatively trivial using some CSS. For example the below will increase the scale of the comments by 25%.
Code:
.cbActivityArticleComments {
zoom: 1.25;
}
Maybe this would work better for you instead? You can apply custom CSS to CB using the below or using Joomla's user.css template file.
www.joomlapolis.com/blog/kyle/18711-template-css-overrides-made-easy