I don't think a CMS plugin should control whether a menu is displayed on the CMS page, CMS plugins should play nice with pages that may have other CMS plugins on them.
If you want a CMS page without a vertical sub menu you can create a skin without <portal:PageMenu and assign that skin to the page.
In supporting pages of a feature like an edit page it is reasonable to hide menus if needed and it can be done. In the HtmlEdit.aspx page for example in OnInit we call a method on the mojoBasePage:
SuppressPageMenu();
Hope that helps,
Joe