@flightDetent:
did it ever occur to you that FCOM is customised? ie different airlines different sops...Some airlines disregards Airbus sops completely and rewrites the procedures. But those have mature pilots and great Flight Ops cultures. It actually makes sense: most of the aircraft manufactures sops are not really thought with safety in mind and, you should know that by now.
The A 320 is especially fitting in the above sentence: for many reasons I have no time to explains the philosophy of SOPS for the A320 were thoughts in a period of crazy mentality that was dominating the airbus certification division in the mid 80s.
Safety was a side benefit of a concept that was trying to extrapolate the pilot from his job.
So: many companies many sops.
Going back to the topic, there is no evidence that doing the after start checklist after the hand signal would add some layers of confort to the safety of the ground crew: it is like saying that we should not be taking off if it is not CAVOK.
So, as you might start to discover there is no advantage whatsoever: by applying the same logic that you bring forward with the checklist after the hand signal, we should never do flight control checks during taxi and, god forbid, Single Engine Taxi Out. Way too dangerous...
More does not necessarily mean an improvement in safety.