How to draw QuickGUI after all overlays?
One thing you could do is set the RenderQueue the GUIManager uses to listen to to RENDER_QUEUE_MAX. This is the last and highest Render Queue. The alternative is for my to change the code a little. Currently I render everything on the renderQueueStarted notification, but I could change this to the renderQueueEnded notification. In this case, the GUI would always render after Overlays. I'm not sure what the default behavior should be. I suppose I could add an if comparison in there and allow the user to choose to display over or under all overlays.
I think this would be GUIManager::setRenderQueueID or something similar. I don't have access to the code right now. Maybe other users can voice their opinions for the second option, regarding moving when QuickGUI renders.
I think that should be configurable.
I thought about this some more, and I think the current design works fine, where you can specify the render queue to render to. QuickGUI Renders in the renderQueueStarted notification, so it actually renders before the given render Queue. If you want the GUI to render over Overlays, set the RenderQueue to RENDER_QUEUE_MAX. (ID might be innacurate) You can still use the last render queue for other objects, so even on max you can write on top of QuickGUI if you want.
I haven't tried this, but if you want GUI both below and above overlays, you should be able to create 2 GUIManagers on different render queues. Of course, there is more to manage at this point, but in theory this could be accomplished.