Clearly the 787 isn't short of electric power sources. There are . . .
• four variable frequency engine starter/generators
• two variable frequency APU starter/generators
• three external AC power receptacles
• one Ram Air Turbine (RAT)
• one main battery
• one APU battery
• three flight control Permanent Magnet Generators
• two EEC Permanent Magnet Alternators
All the other power supplies able to supply enough power for braking require a fuel powered generator when when in flight (Engine Generators or APU). The scenario that others have referred to is the "Gimli Glider" where an aircraft ran out of fuel and glided in for a deadstick landing.
My understanding is that many believe that the RAT won't be able to provide enough power for braking as the aircraft slows upon landing (note: I haven't seen this assumption confirmed or denied by Boeing).
So the question is: Will the FAA/JTSB, etc accept the statistical probability being execeedingly low that a 787 would not run out of fuel and lose a battery on the same flight, and therefore have no wheel braking?