if it is just encoding/decoding ffmpeg can go entirely from stdin to stdout IIRC, provided formats support it. So for example it won't work for mp4 output as it needs to seek to write some headers (again, IIRC) when saving it.
Wouldn't it work if you stream the resulting video as a Youtube Live stream?
You would have to set a stream up in the Youtube creator web page, in order to get a streaming key and with it, a full RTMP URL.
Then in FFmpeg, use "-f rtmp://...your target url" as output. Youtube will stream a live video (which you can hide as "unlisted") and afterwards, the video will remain in the channel as a VOD.
MoviePy is pretty committed to working with files, and I couldn't quickly figure out PyAV pts/dts for the concat even though the files were all exactly the same format.
