mainToolbar.getChildren() should contain your login-item. The problem is that iam not sure if the Renderer will react on your MToolItem.setLabel("username") and updates the label of the Widget that was created for the MUIElement.
Bug 374568 for example shows that the Renderers do not react on every change yet.
The (imho) "bad" way would be to directly manipulate the widget which should work (already tried it for the MainMenu) and should look smth. like this: