The only reasonable facsimile I can immediately think of to the Canadian rules I run with involves an intentional grounding while the QB is in the end zone.
By Canadian amateur rules, B can opt to accept the penalty and the resulting points, or decline and accept the result of the play. Since the points are not scored until a decision on the penalty is made, I will not signal a safety only to then have to take it away if B decides to decline.
I think that's a wise principle to follow here. Since the score doesn't actually take place until the decision is made, don't signal the score until that decision... It's a hell of a lot easier to add points after the fact than subtract 'em.