Sorry, if I'm over lapping anyone here with out reading the whole thread, but this technique has been around for years. I used to teach it at my own Ar classes probably ten years ago and for this very reason. But instead of tieing another system, I would just use a ascender with micro. If they were climbing on a Tauntline or Blakes with out a micro, I would just bring one up a small biner or another micro and attach underneath their hitch.
I wont take credit for ever coming up with it, because most things have already been thought of at one time or another. I will say, that something to the same effect was taught to me by Robert Phillips who is a guru at formulating techniques for ascension and among many other things and I just applied it to this same application.