I think you can just cast the Page, or the PageBase property if you inherit from UserControlBase, to a SystemPageBase to get a reference to the container.
EPiServer Development Team
I have added a new tab to the "EditPanel". And if understand it correctly I can only use usercontrols in this section.
So my question is the following: Is there any way to use the "SystemMessageContainer" to show messages from a UserControl?