Simple Masking/Stencil in the VSE How do you do a basic image mask in the Video Sequence editor?

I have two images that I want to mix together using a third image, where the black pixels will mean showing the first image, and the white pixels showing the second. Like so:



(First and second images mixed using the third image, the mask, resulting in the bottom image)

This is trivial in the compositor, but I have several dozen shots to do this for so I'd rather not make a mess of scenes and compositing. How can achieve this same result inside the VSE alone?
You can use a mask strip modifier:



In the above example scene1 contains a cube, scene2 contains a Suzanne, and mask_sequence is a black and white mask.

The mask modifier can use either a mask or another VSE strip to modify the transparency of the modified strip according to the darkness of the mask. If you set the blend type of the modified strip to Alpha over, the strip(s) underneath will show through the transparent parts.