Maybe it is something along those lines. Like you said the girl in the novel is Japanese, so it wouldn't work if she carried a completely foreign name, but a name that could possibly exist in Japanese as well, and yet wasn't used at that time. So this is only speculating, but since Tanizaki was one of the authors heavily influenced by the West, he maybe came across the name Naomi
somewhere and thought "oh, this is a western name, but it could also be japanese". Else
it would be weird how he descripted it as "if spelled in letters looks like a Westerner", that association must have come from somewhere.
So it could be that he chose Naomi
because it sounded like "Naoko
", but with an ending much less common, making Naomi
the new "modern woman" with a name that was "like a Westener's", while "Naoko
" would be the traditional one.
Apparently he also spelled the name in Kanji as 奈緒美 and not with the typical 直 or 尚, and I'm not certain on this one, but I always felt that 奈 in names was also quite a modern character.
Of course, the author using the name because of it having a double origin is also possible. To be certain one probably really has to research the creation procedure of the novel or find some record of Naomi
as a name used before that date.
Here is another wikipedia article, where they also list Naomi
along with Risa or Erika
as an originally foreign name that is today commonly considered a normal Japanese name.http://ja.wikipedia.org/wiki/%E4%BA%BA%E5%90%8D