I've never had to do it, but i would think you may have to remove the entire oil pump (the massive aluminium casting the full length of the sump) to make it easier to get out. (Make access easier anyway)
I would hazard a guess that you will have more damage that just the broken bolt to worry about though so the engine will have to come apart / out?
What happened when it failed. Did you notice and turn it off quickly or did it stop itself

