John Singer Sargent found a girl willing to pose nude in Egypt, where one might imagine women were expected to be pretty buttoned up.
In The Reading Girl, Théodore Roussel, a Frenchman living in England, painted a popular model from the era, Hetty_Pettigrew, who apparently earned good money posing for many artists. Would her profession have been seen as highly immoral by the standards of the day?
"in Arab-dominated countries"
I can't say with certainty that this particular model was a prostitute, but it strikes me as the most likely way he would have found a nude model in Egypt in the 19th century. John Singer Sargent apparently was no stranger to brothels, as this page about the painting notes at the top.
Prostitutes were basically the typical way that European men gained access to women in the Middle East in the 19th century, with sometimes preposterous results. So for example when Gustave Flaubert was traveling in Egypt he described the erotic "dance of the bee". It was erotic because the performer was a prostitute he had paid and then slept with.
This and other descriptions by 19th century European observers writing about how their prostitutes danced for them created a fundamental misunderstanding of what Arab dance consisted of, among other things.
There were some exceptions. Edward William Lane writing in the 1830s recruited his sister to report on the goings on in the harems, bath houses, and other women-only spaces that he could not be admitted to.
Here is a response to a similar question about prudeness in the Victorian era