workable, but not great
I used the same pinout to add a 3.5mm jack to the original headset end. The problem is the wires used in the cable are very difficult to solder. They are more like a metallized fabric than plain copper wire. And I am pretty good with fine wire soldering in general.
That said, I did get the jack on (though I'm not thrilled with the non-flowed solder joints) but the output level is pretty low. If I plug it into the input jack on my car stereo, the GSM phone noise (which I *hate*) overwhelms the wired signal every time I change towers.