does Guideline 4.3(b) prevent an app that is already approved and live from shipping updates?
Each new submission is evaluated independently of all previous submissions of the same app, and all previous submissions of all other apps from all other developers.
Any guidance from App Review, or from developers who have faced 4.3(b) on an update rather than a first submission, would be very welcome.
By guidance, I assume you mean suggestions for getting the app approved. Sorry, but I don't have any such suggestions. I can tell you that developers obsess far too much over the guidelines. The fact that so many developers have memorized the individual paragraph numbers is amazing, but sad.
They're just "guidelines". It isn't any kind of law, regulation, or policy. Apple can, and will, reject your app without citing any guideline at all.
This is a power relationship problem. Apple has all the power, developers have none. Getting rejected is a signal that you need to do things differently. Trying to beat down a door for 288 accounts is a waste of your time. The fact that they are citing a specific guideline is a suggestion regarding what you need to do differently on your next app idea.