IIRC, you have to create a GL context with an alpha channel and clear the framebuffer with the RGBColor variant of the clear method to set an alpha while clearing. There should be a simple example of how to combine the camera view with a rendering on top, which is a similar task, somewhere here in the projects section. Though it's hard to find for me ATM too, because I'm on mobile right now...
Edit: There's a sticky topic here in the support section about that camera overlay thing...maybe that helps.