Possible to stream/append images into an existing mp4?
Everything is possible!

Let's image, you have a /directory/ per video animation and files like img0001.png, img0002.png. You have 3 ways:


Wait all images, img0001.png, img0002.png ... img0100.png to be in directory, run ffmpeg, get mp4 and remove images (current way)
Write application in C with ffmpeg libraries: wait every single image, decode it in memory, remove from disk immediately and feed to encoder in libavcodec. When you got last image, for example img0100.png, you feed it to encoder and call something like flush() to complete image, so your mp4 will be completed. Drawback of this method is memory consumption - your process will live until all images processed, so if you downloading it long time, process will consume some memory all this time!
You can try to implement same in bash script using pipes:


Here we got any input, list files for example:

`    ls -1 | grep "jpg$" | while read filename; do`


Then unpack images:

`    ffmpeg -v 0 -nostdin -i "$filename" -f rawvideo -s 120x60 -pix_fmt rgb24 - `


And encode it:

`    done | ffmpeg -nostdin -f rawvideo -s 120x60 -r 10 -pix_fmt rgb24 -i - -s 320x240 -vcodec libx264 -y res.mp4`


I think, you got idea.