Hey Mark
I don't think you can achieve this with an Ext Matte (I'm going to check this later again out of curiosity)
The Match Move OFX could have been your friend here.. but it still doesn't support alpha properly nor it has Screen blending
You can try doing it using 2 layers (timeline tracks)
1. On tracked shot (layer 1) - Use point in classic stabilizer. Strong on 100. (do it on an ungraded version)
2. In gallery right click and select Apply Grades using: Keyframes Aligning Start frames..
3. Grab a still
4. On target shot (layer 2) - apply grade.
6. In stabilizer make Strong -100, then hit stabilize.
7. Don't forget to bring back right version on layer 1