CMata_Intel, of course a lot of baud rates can be set (and thanks for the link to the manual never the less), but the one I need for DMX - something close to 250k - can't be set straight forward, as it is not a standard baud rate. Is is also not a standard baud rate on most *nix systems. Just test the commands you proposed! "stty -F /dev/ttyS0 250000" doesn't work, because it is not a standard baud rate, the error is "stty: invalid argument '250000'".
If you are curious, arduino-cores/TTYUART.cpp at master · ozel/arduino-cores · GitHub, defines all the supported baud rates. Which is why I started changing the code there, as an alternative to mess with setserial and stty calls on the command line.
I might publish an example sketch as a project. But as I wrote, the relevant code lines I used to create valid DMX packets are all documented above.