This would be a great addition.
I guess it could be kind of a combination off "chroma key" and "image overlay". Also PS layer modes like multiply, darken, lighten etc. would be a great addition. This stuff is standard in every video software. And i guess its not hard to do.
Video processor and video in Reaper has such a huge potential. More people should code presets. I tried to write and change some presets but as a noob its pretty hard to put something together. Even the Reaper internals couldn't help me.