[ConKbot of Doom]

Since Asycronous serial is relatively easy to implement on the RC, what I would do is setup an Async. Serial connection to another PIC, (a 16F would do) and have that be the master for an I2C bus, along with providing a few more analog inputs, and GPIOs. A 16F873 or 16F876 has both an USART and a MSSP (Master Synchonous Serial Port) as opposes to a plain 'ol Synchonous Serial Port. If you don't want anything other than the Serial-I2C bridge, then you could have it simply get a byte from serial, output that, and then when it recieves a byte, it sends that back over serial, though you may want to add some control features to the bridge so you can do exactly what you want to.

But check the 18F8520 datasheet first, if your lucky there may be more than one SSP module, and if you are really lucky, those IO pins on the second SSP will be on the digitial-IO header.