maybe you should break this down into a flow chart ?
what is sounds like to me is that you have two micros. one is the master and the other is a slave(a lot of sensors)
on the I2c bus, you send our a request to poll I2c address 0, ( a simulated temperature sensor )
the arduino slave needs to recognize that it needs to be sensor #0 and to output a signal that would simulate a temperature sensor.
then, the master wants to talk to sensor #3 a simulated pressure sensor
so the call on the I2C buss to sensor #3 is made, the slave sees the call to sensor #3 and then outputs a value that would simulate what the pressure sensor would send.
sounds like a StateMachine might work.
http://www.thebox.myzen.co.uk/Tutorial/State_Machine.html
or http://www.gammon.com.au/forum/?id=12316
step one)
you need to determine what sensor is called.
step two)
run the statemachine for that sensor.
step 3)
output the values that sensor would have.
in this part, you might be able to connect to a real sensor, capture the data and then just replay it
you could use an analog input with a thermistor or light sensor or some such as a variable to alter the output so that the simulated output was not always the same.