I want to have a shader that drops frames. it's mostly for video playback, not gaming, although for some games maybe a bit of choppiness is what you want.
I'm not any good with HLSL but I'd like to see if I can repeat a frame a certain number of times before I grab a new image for the frame buffer, set a pseudo framerate.

You can't change the framerate, but you could store an old frame and display that for a couple of frames before updating it with a new frame. So yeah, should be possible technically.