Isn't it possible to reprogram the individual nunchuck I2C addresses? Or is that a question of the individual I2C device? From what I read about the protocol you could send out a general call and reprogram individual addresses.
according to the I2C spec the bus will only work if all devices on the bus are powered.
21.2.2 Electrical InterconnectionAs depicted in Figure 21-1, both bus lines are connected to the positive supply voltage throughpull-up resistors. The bus drivers of all TWI-compliant devices are open-drain or open-collector.This implements a wired-AND function which is essential to the operation of the interface. A lowlevel on a TWI bus line is generated when one or more TWI devices output a zero. A high levelis output when all TWI devices tri-state their outputs, allowing the pull-up resistors to pull the linehigh. Note that all AVR devices connected to the TWI bus must be powered in order to allow anybus operation.