I know its been too long but now I am in the I2C too and looking around for Interrupt solutions found this thread.

If I may suggest to use interrupts on PORT B but discriminate in the ISR where that interrupt came from in the first place. If it was by I2C then serve I2C_ISR else serve OTHER_BIT_ON_PORTB_ISR.

Just an idea if it is not tooo late!

Ioannis