There is no variable for doing the IP address, although there is a solution to your problem (as I understand it).
When adding a target to MultiPing, alias the IP Address with a name of your choosing like this:
192.168.1.128 My Target 1
MultiPing will use "My Target 1" for the name throughout, including the alert log files. This means you'll be able to specify the "override" name for that IP so you won't have problems with multiple targets with the same name.
Will this soluation satisfy your need?
Just for information, here are the list of variables for MultiPing 1.01.2:
$host - The monitored host – i.e.: the target that failed.
$year - Current 4 digit year (i.e.: 2003)
$month - Current 2 digit month (i.e.: 03)
$day - Current 2 digit day (i.e.: 08)
$date - Date, 4-2-2 format (i.e.: 2003-03-08) for March 8th.
$hour - 24 hour format, (i.e.: 06)
$minute - 2 digit minute, (i.e.: 02)