But they don't know more than I, so are generally useless at brain-storming

Brainstorming has at least two components: knowing things, and recognizing that a thing you know is relevant to a situation. People inside your head might not be helpful at the former but they might be helpful at the latter, thanks to the brain's useful ability to mimic other brains.

I think Eliezer might have been inspired by internal family systems, which means this might be more useful at being effective than it sounds.