Indeed, and i think this is a great idea......we can use filter (ie. allow per content notification) and also, put a feature which sends a msg when the person gets online.
Edit: After readin Key's post....im convinced its not possible....(plz tell me im wrong)
No wait....it is possible
you can add your self and then wait for the person to come online....and when s/he does, send the SMS to your self...(of course, you have to subscribe to Messenger and have your self added to your list)
Is there any other way?