je.f
If at all possible, then squawk 7700. An emergency descent is probably one of the very minute number of emergencies though whose 'Immediate Actions' makes setting the squawk immediately fairly difficult.
I personally would like to see 7700 for every emergency, even medical PANS etc, because it does alert other people to the fact that there is something going on.... and if nothing else, it may prevent the controller concerned being called on the telephone for something less important thus taking away their attention momentarily.
I can't think of any radar system that interperets a fast descent as an emergency..... if that was the case, the military jets that are in abundance all over the place would be forever triggering it!!