I was just looking at the frame, which is conveniently bare. There appears to be an internal carbon shell around the BB spindle. I don't know if that can be easily removed and put back in. I do recall once popping out one of the external flanges of that shell when I was pulling a bearing, which makes me hopeful the shell comes out.

If I were to do this project, I'd want to go with an internal battery too, no point in drilling holes in a frame and then bolting stuff on the outside of it.