Unfortunately, trying to take out the Deathlords didn't work out so well. Turns out their UF shouts can blow you off your dragon. Oops. My second time around I tried to use a ward. It somewhat worked the first time, but caused a strange glitch where my char was now running on the dragon's neck rather than seated. The next time it didn't work at all and I was blown off the dragon again.
On the whole I would advise against fighting Deathlords from the back of dragon.