What you are trying to monitor is the site collection itself, then, but if one site collection is not responding, shouldn't all of the ones hosted by the same web application/content database go down as well? Have you experienced this?
What you could do is create separate LTM pools with the same SharePoint servers as members, then use an iRule to assign traffic to the required pool based on the host name in the request. We have an EAV monitor that supports SNI. You could create one instance of the monitor for each FQDN and assign it to the pool for that FQDN. Here's the EAV script:
!/bin/sh
These arguments supplied automatically for all external monitors:
$1 = IP (nnn.nnn.nnn.nnn notation)
$2 = port (decimal, host byte order)
This script expects the following Name/Value pairs:
HOST = the host name of the SNI-enabled site
URI = the URI to request
RECV = the expected response
Remove IPv6/IPv4 compatibility prefix (LTM passes addresses in IPv6 format)
NODE=`echo ${1} | sed 's/::ffff://'`
if [[ $NODE =~ ^[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}$ ]]; then
node is v4
NODE=${NODE}
else
node is v6
NODE=[${NODE}]
fi
PORT=${2}
PIDFILE="/var/run/`basename ${0}`.sni_monitor_${HOST}_${PORT}_${NODE}_sni.pid"
kill of the last instance of this monitor if hung and log current pid
if [ -f $PIDFILE ]
then
echo "EAV exceeded runtime needed to kill ${HOST}:${PORT}:${NODE}" | logger -p local0.error
kill -9 `cat $PIDFILE` > /dev/null 2>&1
fi
echo "$$" > $PIDFILE
curl-apd -k -v --resolve $HOST:$PORT:$NODE https://$HOST$URI 2>&1 > /dev/null | grep -i "${RECV}"
STATUS=$?
rm -f $PIDFILE
if [ $STATUS -eq 0 ]
then
echo "UP"
fi
exit
Cirrostratus