beeblebrox wrote:This sounds almost like the way the vedic priests would've viewed the kamma, to explain the castes. The Buddha basically turned this onto its head, making it possible for different kinds of people to turn noble (enlightened), much to the priests' amazement.

I think you misunderstood me. I'm not saying that you can't change your lot, that birth is all there is to kamma. What I'm saying is that from conception until birth, kamma has little influence on how a new being will look, how smart they will be, that kind of thing. The shape a being will take is dependent on their parents (more specifically their genetic code, and to a lesser extent, factors such as the mother's diet). One's DNA is not rewritten after conception to make for a more or less fortunate rebirth, nor is the unborn child capable of making any conscious decisions to steer this process, or even capable of knowing what's going on (as the 6 sense bases are still under development).
So the most significant kammic contribution to the physical properties of a newborn has to happen in the previous life(s), before there's any connection whatsoever with the new body, and this can only be the case if, at death/rebirth, there's in principle multiple rebirths possible, of which only one is chosen, based on past and present kamma at that time.