Variable in custom HTTP monitor

I'm not sure exactly what information you need for each monitor, but if you were able to specify it in the pool member description field, you can then use in your external monitor script a tmsh command to extract out that description for the monitor calls. In the script (standard template):

 Remove IPv6/IPv4 compatibility prefix (LTM passes addresses in IPv6 format)
IP=`echo $1 | sed 's/::ffff://'`
 Save the port for use in the shell command
PORT=$2

So you could use this $IP:$PORT information with a call to tmsh list ltm pool and pull out the information necessary to make the monitor call itself to test up/down status.