After reading that thread, I would prefer that we disallow BB tuning. There's one big caveat though, and that is that I think we should be certain that the values displayed in the replay are actually correct, all the time. If they are, but we haven't done the homework to make sure, people will use the fact that we aren't sure to argue that they aren't correct.
Also, since the additional effort required to police this would be a major concern, why don't we integrate this into our racing complaint process? No one is probably better qualified to suspect a person of adjusting brake balance than whoever spent several laps right next to them. That way, if people think someone else is messing with the dial, they can check their own replay, and (hopefully) only lodge a complaint if they actually see a violation. I think that would minimize the impact on the stewards.
Regarding your point Ap, I take the position you alluded to in wondering what 5/5 actually means. I think PD intended for the default settings (and 5/5 is always the default BB setting) to represent the stock configuration of each car. I doubt 5/5 means even front-to-rear balance. That said, this is PD we're talking about, and who knows