Talking about Windows in general, it is not possible to guarantee that a window will be on top of all other windows, because if you have 2 windows that have this property, one of them will be on top of the other. As you will see in the post below, you have 2 categories of windows and z-order:
http://blogs.msdn.com/b/oldnewthing/archive/2005/11/21/495246.aspx
Assuming you are ok with something on top of all your windows, in WEL, you can use the Win32 API to achieve that by using WEL_WINDOW.set_z_order ({WEL_HWND_CONSTANTS}.hwnd_topmost).
In Vision, you can also do that by using EV_POPUP_WINDOW and calling `disconnect_from_window_manager' on it.