Here is a story of a B-17 crew dive bombing the Japanese after the Philippine invasion.
Dive bombing a B-17
Very interesting, but I can't help but notice they knew it was a near-suicide run and would irreparably damage the wings and frame of the craft. I also can't help but notice the dive was not the point of bomb release, but a rapid descent to a near-level dropping altitude at 2000 feet.
I think in terms of gameplay and what you can police, it's a tricky subject. I think there are some airframes that were much more limited to level bombing than other airframes. B17s had a wider bomb bay, for example, so what limit should be placed on them? 30 degrees? B-25s have a narrow and tall bomb bay, more stuff to "hit" on the bomb's way out. How should it be limited? 20 degrees? We don't have documentation on EVERY plane. It's problematic to the point of becoming arbitrary in how it's assigned to some planes. We also had so much weight and stress on a P-47 lugging wing bombs or DTs that they couldn't do anything more than bank and fly level without fear of damaging the airframe critically. We have no such problems in-game at the moment. So if you implement bomb restrictions and realistic (semi-realistic?) behaviors, what about all the other planes?
Ideally, it would have something coded in, but realistically it could be a very intense undertaking that would leave a LOT of people upset and angry about their new limitations. It's not an easy fix.