In A/UX, some of flow control tty settings such as 'modem', 'dtrflow', and
so on are maintained across last close. This means the flow control
settings are "remembered" when a tty device is closed and reopened again.
However, other termio characteristics like software flow control (ixon,
ixoff), speed (50, 75, 1200, and so on), parity (parenb, parodd, and so
on), input modes (ignbrk, brkint, istrip, and so on), local modes (echo,
echok, and so on) are set to the default settings when the tty line is
closed. The default setting for software flow control on tty0 is off.
In order to make the 'stty' settings affected and still keep the tty port
open put a sleep statement, 'sleep 99999' for example, after the 'stty'
command. In your case, you may do:
(stty hupcl ixon ixoff; sleep 99999) < /dev/tty0 &