Running something from the startup rc scripts never worked because they
are not associated with a terminal, so I couldn't do anything that
required any fancy terminal I/O.
So what I did instead on linux was make my own copy of 'getty' (the
program that starts the ttys) that I modified to run my copy of tn5250,
and then configured my system to start the terminals using my special
copy of getty. This worked very well, and if the terminal session got
killed somehow, the init process would automatically restart it, so that
was helpful, too.