This wouldn't need an existing IntegraB.
My original plan was to make a 'new' IntegraB with more modern / lower power components. I have started looking at the I2C RTC as an upgrade solution for the CDP6818E in the original IntegraB. The IntegraB IBOS rom would need to be adapted to read / write to the I2C RTC instead of the CDP6818E. That would be doable, apart from the fact that it's already using most of the available 16k ROM space for all the existing functions - not sure there's space to squeeze in the I2C driver code without having to remove other functionality.
What would be a lot simpler to develop is a stand alone I2C RTC module for the beeb using the same Sheila I/O address space that the IntegraB RTC uses. It would plug into the 6502 socket, and would have the necessary address decoding circuitry. I would also need to create a standalone ROM to interface with the I2C RTC. I might develop something like that.
However, if you're already in the process of developing something similar...