Jan Rychter released a tiny library that implements an I2C master on MSP430 devices that only have the USI module on GitHub:
I’ve released a tiny library that implements I2C master functionality for MSP430 chips that have the USI module (MSP430G2412 or MSP430G2452 are ones that I use often).
The code is on GitHub, it is MIT-licensed, so you can do whatever you want with it.