Thanks - that set me in the right direction. Since the item I want to hide is in the second level of the rad strip, I had to find the parent, and then remove the child from the parent. I'm posting the code for anyone that may be interested. If this wasn't just a one-off, it would probably make sense to create a recursive function that could walk the tree, as this isn't a very elegant solution...
if (UserManager.IsCurrentUserInRole("custom role"))
RadTab item = MainMenu.FindTabByText("Administration");
if (item != null)
RadTab child = MainMenu.Tabs[item.Index].Tabs.FindTabByText("Tools");