In the case you cited above, I'd say (and I could be wrong without knowing more about this character) is that she's drawing linguistic attention to the "woman doing a man's job" bit. Is this set in present day? If it's set earlier in the 20th century, I don't know that it would be grammatically incorrect for her to say "executor," and if she was simply a pedant, she would probably have just used "executrix" without the "or" clause. Today, using something with an "-rix" suffix would be seen as highly sexist by most people. (Notable excepting is "dominatrix", and they aren't exactly talking about in polite company). Drawing attention to the linguistic distinction could be a way of showing that yes, she knows that the executor is a woman and that there are people who care about that sort of thing. Or perhaps, even, that she cares about the distinction.

Keep in mind that I'm well aware of my own ideological biases when I do any sort of literary or linguistic analysis. So it could very well be that some other interpretation makes more sense than mine. Also, without going to look through textbooks upstairs, I'm a bit sketchy on language use in earlier decades.
-----
inspiritation: the effect of irritating someone so much it inspires them to do something about it. --BuggEye

To take toxicfur's analysis from another angle, the tendency to deprecate the female forms of certain title-nouns (even more common ones, such as women in the acting profession being "actors" rather than "actresses") has a certain agenda, being to blur the gender issue or treat it as irrelevant. Perhaps her statement here (which, it is true, would probably only be made by someone who's something of a traditionalist, and aware of the alternate form) is something along the lines of: "I'm female, and this is my role. Why obfuscate it?" It's been a long time since I read Lot 49, so I can't remember what agenda it came with.

The other clue, I think, is in the name Oedipa -- not common in any decade, I'd wager, and obviously a derivative of a much more famous, male Oedip__. Someone who's carried that name all her life would be acutely aware of male/female inflections, as well as the various roles that one might be expected to play from either side of the gender line. One might be so hyper-aware of all these things, I suppose, that one becomes almost bored with it, and find these linguistic games about as stimulating as a Tupperware party with an over-kirsched fondue.

--
"If we build it, will they come, and what will they do when they get here?" -- iGrrrl