[MrTick]

The custom-login is executed by /etc/upstart/console.conf:

(...)
  # This feature is introduced from Zelda platform. For older platforms, we keep the login unlocked
(...)
if [ "$prodVersion" == "0" -o "$unlockedKernel" == "true" -o "$secureCpu" = "0" ]; then
    #Unlocked Prompt if: unlockedKernel or not prod or not secure cpu
    exec getty -L $BAUD /dev/$UART 2>> $STDERR
else
    exec getty -L $BAUD /dev/$UART -i -n -l /etc/upstart/custom-login 2>> $STDERR
fi

Originally Posted by knc1

there are tools to graph or trace the upstart sequence

I'll check this out, initctl2dot seems to be a good candidate.
Unfortunately I've left my Kindle at work so I need to wait with experiments till tomorrow