Как я изменяю гостевое сообщение предупреждения сессии

Я рекомендовал бы, чтобы Вы создали свое собственное диалоговое окно и отключили по умолчанию. См. это учебное руководство для руководства.

Редактирование (пример кода):

$ cat /etc/guest-session/auto.sh
TITLE='Temporary Guest Session'
TEXT="Hello my Dear Guest. Hope you'll enjoy Ubuntu!"
{ sleep 4; zenity --warning --no-wrap --title="$TITLE" --text="$TEXT"; } &

Источник этого текста является следующим сценарием:

/usr/lib/lightdm/guest-session-auto.sh

необходимо будет изменить его для изменения сообщения, поскольку нет никакого способа настроить установку $TEXT:

#!/bin/sh
#
# Copyright (C) 2013 Canonical Ltd
# Author: Gunnar Hjalmarsson <gunnarhj@ubuntu.com>
#
# This program is free software: you can redistribute it and/or modify it under
# the terms of the GNU General Public License as published by the Free Software
# Foundation, version 3 of the License.
#
# See http://www.gnu.org/copyleft/gpl.html the full text of the license.

# This script is run via autostart at the launch of a guest session.

export TEXTDOMAINDIR=/usr/share/locale-langpack
export TEXTDOMAIN=lightdm

# disable screen locking
gsettings set org.gnome.desktop.lockdown disable-lock-screen true

# info dialog about the temporary nature of a guest session
dialog_content () {
    TITLE=$(gettext 'Temporary Guest Session')
    TEXT=$(gettext 'All data created during this guest session will be deleted
when you log out, and settings will be reset to defaults.
Please save files on some external device, for instance a
USB stick, if you would like to access them again later.')
    para2=$(gettext 'Another alternative is to save files in the
/var/guest-data folder.')
    test -w /var/guest-data && TEXT="$TEXT\n\n$para2"
}
test -f "$HOME"/.skip-guest-warning-dialog || {
    if [ "$KDE_FULL_SESSION" = true ] && [ -x /usr/bin/kdialog ]; then
        dialog_content
        TEXT_FILE="$HOME"/.guest-session-kdialog
        echo -n "$TEXT" > $TEXT_FILE
        {
            # Sleep to wait for the the info dialog to start.
            # This way the window will likely become focused.
            sleep $DIALOG_SLEEP
            kdialog --title "$TITLE" --textbox $TEXT_FILE 450 250
            rm -f $TEXT_FILE
        } &
    elif [ -x /usr/bin/zenity ]; then
        dialog_content
        {
            # Sleep to wait for the the info dialog to start.
            # This way the window will likely become focused.
            sleep $DIALOG_SLEEP
            zenity --warning --no-wrap --title="$TITLE" --text="$TEXT"
        } &
    fi
}

# run possible local startup commands
test -f /etc/guest-session/auto.sh && . /etc/guest-session/auto.sh