Shoutbox

Meh, it's just a suggestion but if a script were made, then one possible idea is everytime you speak to a contact, add their email to an array (assuming they don't already exist in it of course.) You could then use whatever method to call a function that sends the message to all the users (still online) in that array before signing out.

This way at least you aren't sending messages to people you haven't spoken to, yet you can still close chat windows and be assured that those contacts will get your goodbye message...