Quote:
...and mIRC offers no way to safely generate variable names.


Since the variable is unloaded as soon as you call the identifier, just give the variable a messy name, eg:

Code:
strcpy(data,"set %__dllname__functionname_1032842 x,y,z");


It's unlikely a name like that will be in use, you could also make a mIRC identifier to return a free variable name:

Code:
alias freevar {
  var %r = $rand(9999,9999999999)
  while ($var($eval(%r,2),1)) { %r = $rand(9999,9999999999) }
  return %r
}

New username: hixxy