xf86-input-wacom uses systemd to trigger a service
/usr/lib/udev/rules.d/wacom.conf
SUBSYSTEM=="tty|pnp", KERNEL=="ttyS[0-9]*", ATTRS{id}=="WACf*", TAG+="systemd", ENV{SYSTEMD_WANTS}+="wacom-inputattach@%k.service"
I think changing it to something like this should work.
SUBSYSTEM=="tty|pnp", KERNEL=="ttyS[0-9]*", ATTRS{id}=="WACf*", RUN+="/usr/bin/start-stop-daemon --start --quiet --pidfile /run/wacom-inputattach-%k.pid --make-pidfile --background --exec /usr/bin/isdv4-serial-inputattach -- /dev/%k"