First place I would go is to mine the the HF NAT-A&B archives after 1530Z to see if I could get the initial call to ARINC declaring the emergency and informing of the altitude change. Being over three hours out from Barnes (where they landed) they were probably in Gander space, but maybe not since they would have slowed down significantly at <10k' and perhaps added an hour to the meter. Maybe one of the military guys can suggest the call sign that might have been in use... probably based at Dover, possibly REACH or ROGUE, which are among the C-5's out of there.