Answer

I haven’t checked this thoroughly right now, but it is commonly assumed that these prohibitions apply to any woman who is not his wife (even an unmarried woman), and certainly to someone forbidden to him (even if she is not one of the biblically forbidden relatives). Regarding a menstruant woman, this is explicit in the Talmud, and there are opinions that a menstruant is not considered one of the biblically forbidden relatives (see Rabbi Ovadia’s introduction to the book Taharat HaBayit).
It can be explained in two ways:
1. True, the accessories of forbidden relations are a distancing measure from forbidden relations, but even lighter intercourse prohibitions have distancing measures, which themselves will be correspondingly lighter (that is, they would not be considered closeness to forbidden relations, but rather closeness to a prohibited sexual act).
And the explanation is that the distancing does not stem only from the severity of the prohibition (which exists only in the case of actual biblically forbidden relations), but because of the likelihood of stumbling into the prohibition (even if it is not severe). Sexual desire is strong, and therefore the likelihood of stumbling in all intercourse prohibitions is great, so even with prohibitions that are not biblically forbidden relations, it is important to distance him from it.
2. Closeness to a woman forbidden to him arouses forbidden thoughts, and is therefore prohibited like closeness to biblically forbidden relations. This itself can be split: whether the thoughts are about the very woman to whom he is drawing close, or whether it will arouse thoughts about women in general; in that case, perhaps this would indeed be closeness to forbidden relations and truly their accessory prohibitions.