I've been at >75deg N on pitch black nights with no moon, I often ponder the consequences of a total nav failure.
WX Radar ground mapping is not a bad idea, assuming the ATT info hasn't dropped off with the IRS's which might confuse the WX Radar tilt system and disable it. (probabaly on an airbus, Boeing would no doubt go to 'raw tilt mode').
Most hand held GPS units dont work inside airliners ..... otherwise I would carry one!
The answer to this I am sure relies on last known position and earths rotation to calculate heading changes, the problem of True and Magnetic and huge variation changes is what is going to cause problems .....