resend headers NUT formats
Although probably not best practice, this seems to do the trick:

Take a NUT file, same codec and dimensions;
ffmpeg -i nutfile1.nut -frames 2 -c:a copy -c:v copy -f nut nutheader.nut

And then a little piping hijinks:

nc 192.168.1.161 6600 | cat nutheader.nut - | ffmpeg -i pipe:0 -c:a copy -c:v copy -map 0:a -map 0:v -f fifo -fifo-format nut -drop_pkts_on_overflow 1 pipe:1 | nc -L -p 6601

You get a 'discontinuous DTS' when you connect the first time and headers are duplicated, but whatever.

And you get one 'packet damaged' when you start the standard input after the headers, but it immediately and with extremely low latency resynchronizes.