I'm just an "armchair engineer" on this subject, too. All I know is what I read. What you say certainly sounds plausible, at least on paper, and perhaps in the future, EV battery packs will be made that way. It would add an extra layer of complexity because then the BMSs would have to balance charging/discharging not only between the different strings of cells in the module, but they would also have to balance between the different BMSs. I don't see anything there that appears especially difficult to control or program -- not moreso than what BMSs are already doing -- but then I'm not an electrical engineer, and it may be more difficult than we realize.