I know this senario must be a nightmare in the cockpit so you've lots to do but don't forget that many ATC radar centres these days have capability to display the groundspeed on the radar so can relay this. We can also ask other a/c in the sky what indicated they are flying and how this relates to their current groundspeed for conversion sake too (taking windspeed/direction into consideration). Its something I've actually simulated as a radar controller too with a professional pilot acting as the pilot in trouble and both sides learnt a hell of a lot as to how ATC can be of use. We do have many tools to play with these days..........