ok, when I think about it seems logical to me now
You can't do this directly as you would be both reading from and writing to the same GPU-side resource at the same time, so you have two main options.
Of course I would read the background, warp it and then render the front of the object on top of it. Doing that at the same time can´t work on a logical level... did not quite see that before
I guess I will go with the framebuffers then, no problem at all when post effects are applied (at some point in the future). They need offscreen buffers anyway.