Here's a guess: upload from a browser running on Linux, and create a FUSE filesystem that blocks read() calls until you've generated the video.
I tried it with a named pipe ("mknod /tmp/foo.mp4 p") and it gave me an error, but before the error, the web page blocked during upload, but it showed me the URL that will be used (before I gave it a single byte of data).
I tried it with a named pipe ("mknod /tmp/foo.mp4 p") and it gave me an error, but before the error, the web page blocked during upload, but it showed me the URL that will be used (before I gave it a single byte of data).