Switching to a 20mm through hub shouldn't cause any interference with your brake caliper.
A lot of chain devices can be fitted behind the bottom bracket cups. If you're using Hollow Tech 2, i.e. external BB cups, then the backplate of the chain device simply replaces the one black spacer currently between the cup and frame. Older style BBs can still do the same thing, but I am not as sure about chainline and spacing. I've run one with an older style BB in the past and it all sort of worked, but my chainline wasn't great.
IME, the easiest devices to set up and get working using behind a BB cup are the Gamut devices (come to think of it they are the easiest to set up full stop). You can get adapters for all e13 devices as well. They look like this and usually come with the device when bought new.