I have tried to use my version of PLATOTERM in MAME, and am not getting meaningful output from the terminal (it works on real hardware):

I am running a copy of tcpser: (http://github.com/tschak909/tcpser) with the following parameters:

tcpser -v 25232 -i "&K0" -s 9600

and connecting via mame with:

mame64 mindset -m1:rs232:rs232 -bitb socket.localhost:25232 -flop1 plato.img

and have also tried running tcpser on a comport loopback (which is how I usually run it):
tcpser -d /dev/ttyS1 -i "&K0" -s 9600

mame64 mindset -m1:rs232:rs232 -bitb com1 -flop1 plato.img

And the disk image is here: