You can use ffmpeg losslessly on the decode side as well, but the only interfaces I could find to write to are v4l2 and fbdev, neither of which seem to work pro

You can use ffmpeg losslessly on the decode side as well, but the only interfaces I could find to write to are v4l2 and fbdev, neither of which seem to work properly under X11. (The obvious candidate, x11grab, a: reads, not writes b: the entire screen, not a specific window.) So you'd need something else to pipe the raw video to (ffmpeg can dump the audio into alsa). You could also not decode it, and just demux the video and pipe that. Depending on how it reads/decodes files, a image viewer that displays GIFs might update in close to real time as it reads from a pipe that ffmpeg is demuxing into, and might not leak memory by keeping the overwritten frames around in case the GIF loops, but those both seem unlikely. (As the saying goes: if it only works the way the manufacturer intended, it's defective.)