One extra consideration you may want to consider is that any time you dive with compressible displacement (i.e. padding or neoprene), you create buoyancy at the surface requiring more weight, but at depth the compressible stuff compresses and now your extra weight requires extra air in your BCD.
By itself I don't imagine a back plate pad would create a safety issue, but if you dive that, plus a wet suit, plus steel tanks, etc., you create a situation where a wing failure at depth can have catastrophic consequences (which is why some people choose double wings for diving wet suits with heavy rigs and others choose to dive dry even in warm water).