I suggested this on the initial AF447 thread, and didn't get much response, but I don't see why it would not work:
As long as you have a valid IAS, you systematically compare it to GS using GPS and compute wind speed and direction from that. You store this value. If the IAS fails, you use this stored value to correct the GPS-derived GS to get an estimated AS.
Except in the case of severe wind shear, why would this not work?
You could present the estimated AS as a range, with some degree of uncertainty, incorporating previous variations in wind speed, if any, or just an uncertainty factor (perhaps growing over time) to account for possible changes in wind speed.
I've suggested the same. Ok, flying into a weather front - as they did - could mean rapid changes in both direction and speed of winds. Yet having an advisory reading on your display, perhaps showing GS, IAS and their difference side by side in colored numbers could be, I'm imaging, a handy tool when things start to go wrong.