int itemIndex = toolbar.AddItem("esriControls.ControlsMapFullExtentCommand", 0, -1, false, 0, esriCommandStyles.esriCommandStyleIconOnly); IToolbarItem item = toolbar.GetItem(elementTool);
I didn't see the abillity to toggle visibility of a ToolbarItem in the API, other than removing the item from the Toolbar given an item index.
Did you look at ICommandItem? The delete method just removes the command from the commandbar. The docs indicate you can add that command back at a later time.