Dividend Tax and Director Tax Returns

I raised a separate question over HMRC issuing tax returns to a director going back to 2011-12 (which didn't receive a single response because of the changes to ICAEW ION, that is another story) but it now occurs to me that this might become a widespread issue because of the Dividend Tax?

If a director is not already in self assessment but receives dividends of more than £5,000 for 2016-17 they will have a tax charge which will have to be settled through self assessment and they will have to register for SA.  However at the point that they register for SA might HMRC say 'hold on a second, you should have been within SA before' and issue additional tax returns for the previous four years?  That is essentially what happened to my client, but the trigger in my case was the decision to request that the dividend tax for 2016-17 be coded out.

So does HMRC have a policy on this?  What should we expect?