I forgot to say that the output of ffmpeg is a multicast network stream to be used with http://www.piwall.co.uk/ , if anyone cares.
So, in my case, Millumin to Syphon to Processing to FFmpeg to Big-Video-Wall-Made-Of-Lots-Of-TVs-Running-On-Raspberry-Pi
Latency is about 0.4 secs on my rig… Could be worse…
Yay!