I see...

It can be done, you will need to maintain your own IAL in a hash table.

Hashtable
==========
Key format: *!user@address
Value format: (All nicks separated with a comma token)

Key example: *!me@mirc-dll.com
Value example: Naquada,NaquadaServ,NaquadaBomb

Code:
; $1 = *!user@address (from $mask($address, 0) in join event)
; $2 = $nick (from $nick in join event)
; returns nothing
alias watch {
  var %existing $hget(WatchIAL, $1)
  hadd -m WatchIAL $1 $addtok(%existing, $2, 44)
}

; $1 = nick1
; $2 = nick2
; call as identifer, returns $true if nick1 and nick2 are from the same user/address
; returns $false otherwise
alias isSameAs {
  var %Index = 1
  var %match = $hget(WatchIAL, $hfind(WatchIAL, $+(*, $1, *), 1, w).data)
  while (%match != $null) {
    if ($istok(%match, $1) && $istok(%match, $2)) return $true
    inc %Index
    set %match $hget(WatchIAL, $hfind(WatchIAL, $+(*, $1, *), %Index, w).data)
  }
  return $false
}

Note: I wasn't exactly sure how you wanted to use this information, also I havent actually tested the code, let me know if you have any problems with it...

NaquadaBomb
www.mirc-dll.com