I just don't see the point in using a global var in something like this. Look into $awaymsg instead of using a global var. An yes away messages are annoying.

alias AFK { 
  ame is Away $iif(!$1,Gone,$1-)
  away $iif(!$1,Gone,$1-)
}  
alias ImBack {
  ame is back from being Away - $awaymsg
  away
}