Personally, I wouldn't worry about it. I would happily run a flash expecting 9 volts on 11.1 volts. A resistor won't work to drop the voltage as the voltage drop across it will vary with current. If you set it so it drops 2 volts at the start of the charging cycle, when it is fully charged it will hardly drop any voltage.

Three silicon diodes in series would do a better job dropping about 0.7 volts each.