Fair enough. You realize the 'nut' of this particular problem is capturing the 'free/busy' state. Capturing state requires a two things, a 'probe' (something which measures) and a 'policy' (something which looks at the measurements and produces a finding).
Fortunately, in your particular case, the challenge of creating a probe which can measure 'busyness' answers your question (which is effectively 'No' btw). Chat programs work because they limit the problem to 'are you currently in chat' and can probe for that but without instrumenting your friends you are stuck trying to infer their 'busyness', and many people that kind of probing is considered none of your 'business' (pun intended).
A limited (and its limited because it has to be proactive) conceptual idea is an app with a button that says 'bored' on your phone. When you press it, it would tell you how many of the friends you are 'following' have pressed that button in the last say 30 second, minute, or 10 minutes and offer to SMS that set. The downside is that it will not include folks who are doing something but they would rather be doing something else.