Make Live Target transparent

I have a screen image that starts off obscuring the device camera, but will gradually disappear as the user records. I'd like to show a preview of what's "behind" the screen image so the user can compose themselves properly by rendering a semi-transparent view of the device camera on top of the screen image, however I don't want the preview to be captured. 

My plan was to follow this guide and have it render to a Live Target. It works when I render to the main render target, but when I switch to the Live Target, it breaks--i.e. either shows the device camera image with no transparency, the clear color for the Live Target, or black. Any idea what's going on? 

Many thanks!