[Francis Baconnaise]

I have an idea…

What if you use the curl feature in FCP, duplicate your media and stack them on V2 and V3, leaving V1 empty. Then with the copy on V2, play with any of the color, gamma, or brightness & contrast controls until you have what is essentially an alpha channel. Then set the composite mode on the V3 copy to Travel Matte Luma.

The V3 clip will use what lies directly beneath it as a Matte. White shows, black is transparent and passes through to whatever lies on V1.