Overlay a live WiFi camera image I am using a WiFi endoscope camera with an Acer Android tablet, for woodturning hollow forms. The camera points to a cutter on the same structure as the camera so that the camera is always focussed on the cutter.

I need to be able to superimpose a graphic that I have created over the live image of the cutter. Essentially the image is of the cutter, but with a margin drawn around it.

I have tried many apps that allow overlaying one saved image over another, but none of them work with a live camera "feed". I found one that does allow an image to be superimposed over a live camera feed, but it will only use the camera feed from the camera on the tablet.

Can anyone here help me with this?
great, I got it . This is easy and possible, SO first of all i need to know the camera you are using to record or live preview your work ?

You need a good high resolution camera , which can zoom at that detail what was shown in your Youtube link . 
Secondly if the camera has live streaming / mirro link options that will be cool , cause you can connect to TV  / Mobile in android and get the camera  live streaming in you mobile or tablet. 
Your tablet with android 6.0 is good enough . 
The important thing is you Camera that will record / live stream when you work. 

Secondly , you can do this wirelessly and wired way. 
Wireless ,if you cam has a wifi direct / mirrocast , airplay like features , wired with it supports direct camera raw data to pc streaming ...

Hope this helps..