The attiny85 has USI also
USI is Universal Serial Interface.
How this is done is beyond me but I'd like to know how you got on?
Page Arduino Playground - USIi2c
Extract
I2C (master and slave) on the ATtiny85
Overview:
The ATtiny85 microprocessor is an 8 pin chip with 6 (max!) I/O ports. Using an I2C bus greatly expands the possibilities of what you can do with this chip.
The ATtiny85 (and it's cousins) does not have I2C (or SPI) "built in". Instead it has a Universal Serial Interface (USI) that can be used to facilitate I2C and SPI.
After a bit of searching, I found 2 sets of code that use the USI for I2C - one for I2C master and the other for I2C slave. I wrapped each in a separate class and had them follow a similar usage as the Wire lib.