A couple of thoughts as to how to achieve this:
- Look at something like the Bus Pirate. I believe it can simulate an I2C device, so it might be able to simulate more than one. You'll need to write some software in something like processing, and it may still be too slow, but its designed to monitor and interface with I2C (Among other things).
- Write your own code. You could still use the arduino (or the ATMega328/ATmega32u4) to write it, but you'll need to probably write your own drivers. Either you'll just need to change it to be able to monitor multiple channels (might not work), or ignore the Wire library and instead bit-bang your own I2C responses out. The protocol is not /that/ complicated with only a single master.
- Use 5 different Arduino boards, each set up as a different device? Not much different than the various parts really, but its a possibility to stop-gap until you get the full device connected.
I notice that you say "network". This implies something more complicated than I expect. The benefit to I2C is it only requires 2 lines for data transmission of up to 127 devices, so its not as much a network as a bus, with everything tied into it. Also, I2C doesn't work very well past 15 ft (probably out of spec at 15 ft as well, but we have it working at 100kHz).