I now see that my answer for the Raavan has some holes, which include:
If the mamzerus comes from the isur of goy, why isn't there mamzeirus from a pnuyah?
Why does the issue of the act making her asurah libaalah play any role here? In other words, why is vlad pnuyah and eishes ish be'ones not a mamzer?
Therefore, I will refine the last post to say a slightly different pshat in the Raavan. The main points are:
1) His girsa is goy v'eved haba al bas yisrael havlad kasher, but he holds that that doesn't apply to 100% of the situations, only 75%; for the remaining 25% he holds it is vlad mamzer.
2) The mamzeirus does not come from the isur of cohabitation with a goy, it comes from the isur of eishes ish! (Bnei Noach are also muzharim on eishes ish). And more specifically, the mamzeirus comes from the woman violating this isur. A pnuyah does not violate the isur of eishes ish because she is not an eishes ish, and be'ones a married woman does not violate the isur.
3) When we say there is no Mamzeirus for a goy, that is for a complete goy. But the Raavan will hold that when the mother is Jewish, the child has enough yichus to have a psul of mamzeirus (somewhat connected to the last paragraph in the previous post).
So: when there is a rape, an eishes ish did not commit any isur, so a child from a goy who raped her will not be a mamzer. Only if she willfully cohabited with the goy will the child be a mamzer. Also, because by definition a pnuyah is not an eishes ish, her child cannot be a mamzer, even beratzon.