真面目に動作確認していないです。 mysqladmin pingの結果から参照スレーブをよしなに切り替えてみる。

switch.sh

#!/usr/bin/env bash

###################################################################
# switch.sh - check mysqladmin ping and rewrite iptables
# Copyright (C) 2013  yoku0825
#
# 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; either version 2
# of the License, or (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 51 Franklin Street, Suite 500, Boston, MA 02110-1335 USA.
###################################################################

PATH=/bin:/usr/bin:/usr/mysql/5.5.31/bin:/sbin
DUMMY_IP="192.168.56.100"
REAL_IPS=("192.168.56.1" "192.168.56.2" "192.168.56.3")

if mysqladmin -h ${DUMMY_IP} -u root -pxxx ping > /dev/null 2>&1 ; then
  exit 0
fi

iptables -t nat -D OUTPUT
for ((n = 0; n < ${#REAL_IPS[@]}; n++)) ; do
  if mysqladmin -h ${REAL_IPS[n]} -u root -pxxx ping > /dev/null 2>&1 ; then
    iptables -t nat -A -j DNAT --destination ${DUMMY_IP} --to-destination ${REAL_IPS[n]}
  fi
done

exit 0