Two quick thoughts on this very interesting topic.
Another category of largely "missing persons" is the figure of the "mother". In this case the individuals are not simply hiding. Instead, the author has "done them in."!
As was discussed on the Tolkien as matricide thread, the motif of the missing/dead mother was a common one in fairy tale, and Tolkien uses this device extensively, whether for reasons of plot or as an unconscious reflection of his personal experience. An extraordinary number of the main characters in LotR had their mothers die. We see future mothers like Rosie and Arwen, and happily married women like Goldberry who have no children, but few who are 'current' mothers. The missing Entwives are, in a sense, also part of this category.
There is one mother/son exception: a pair who do appear at the beginning and end of the book. That would be Lobelia and Lotho! Whoops, that wasn't exactly the loving image I had in mind. I do wish that Tolkien had found a way to incorporate the mother motif into the tale. For example, we are given Galadriel as the "Virgin Mary" but at this point in life she seems far removed from the concerns a mother would have. (Or is she a loving grandmother?? Hmm...) Everything we know about Tolkien suggests he was a dedicated father and a dependable husband. Surely he had images and experiences to draw upon in this respect, even if he had lost his own mother.
But I'd also like to add a word of caution....We should remember that not only are there groups of females missing from the book but also many types of men. You could make a long list of varying types of men ---differentiated by occupation, social condition, geographical location -- who simply don't appear. This is not to say that they necessarily should have appeared. I am sure the author had quite careful reasons for who he put in and who he left out. And yet every so often, I keep looking for someone who's not there.
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Regarding dwarf women....
Esty - My initial feeling about dwarf women was different than yours. I actually envisioned them as craftspeople rather than persons simply desiring to pile up gold and jewels. As artists, they wanted the freedom that being single lends to a person: freedom of schedule and such (which I am sorely lacking at the moment!
). That still means they turned aside from relationships in favor of something else.
I could be mistaken in this, but there is so little written on dwarf women that it's difficult to say.