WRONG! I have come up with a solution. It can be even done with a plus script. When you open a conversation with someone, the script automatically sends a message "hey is <email> online?" Ingenious! The beauty of social networking. Sure this will lead to spam and asking the question to contacts who may not even HAVE that email added, but it would work none the less!
Ironically it also has the same flaws as every other method - your contact could lie to you about the persons true online/offline status 
/ 
