I must say I was surprised by the origin being Trilby and then the coyness over the declaration that she was nude. An uncle of mine was an artist and would collect body parts in a notebook: noses, feet, hands (and other bits) and, like Constable with his stock of trees to put in a deserving landscape, might assemble these as suited the composition. So when I read (I include a relatively long passage as I think the context is relevant):
She said this in English, with an accent half Scotch and certain French intonations, and in a voice so rich and deep and full as almost to suggest an incipient tenore robusto; and one felt instinctively that it was a real pity she wasn't a boy, she would have made such a jolly one.
"We're delighted, on the contrary," said Little Billee, and advanced a chair for her.
But she said, "Oh, don't mind me; go on with the music," and sat herself down cross-legged on the model-throne near the piano.
As they still looked at her, curious and half embarrassed, she pulled a paper parcel containing food out of one of the coat-pockets, and exclaimed:
"I'll just take a bite, if you don't object; I'm a model, you know, and it's just rung twelve—'the rest.' I'm posing for Durien the sculptor, on the next floor. I pose to him for the altogether."
"The altogether?" asked Little Billee.
"Yes—l'ensemble, you know—head, hands, and feet—everything—especially feet. That's my foot," she said, kicking off her big slipper and stretching out the limb. "It's the handsomest foot in all Paris. There's only one in all Paris to match it, and here it is," and she laughed heartily (like a merry peal of bells), and stuck out the other.
And in truth they were astonishingly beautiful feet, such as one only sees in pictures and statues—a true inspiration of shape and color, all made up of delicate lengths and subtly modulated curves and noble straightnesses and happy little dimpled arrangements in innocent young pink and white.
So that Little Billee, who had the quick, prehensile, æsthetic eye, and knew by the grace of Heaven what the shapes and sizes and colors of almost every bit of man, woman, or child should be (and so seldom are), was quite bewildered to find that a real, bare, live human foot could be such a charming object to look at, and felt that such a base or pedestal lent quite an antique and Olympian dignity to a figure that seemed just then rather grotesque in its mixed attire of military overcoat and female petticoat, and nothing else!
It can be read that "for the altogether" meant the full body not simply a head shot or, hands or, whatever. Emphasising that Trilby was delightful in her entirety, discernible even whilst wrapped in a military greatcoat. Not just a pretty face.
This seems to be confirmed later when Trilby says:
"I have sat for the altogether to several other people besides—M. Gérôme, Durien, the two Hennequins, and Émile Baratier; and for the head and hands to lots of people, and for the feet only to Charles Faure, André Besson, Mathieu Dumoulin, and Collinet. Nobody else.
It is later when it is clear she must be thought of as nude - Ingres's La Source, the pose she is copying, is certainly nude:
So that Monday morning found her there, and Carrel posed her as Ingres's famous figure in his picture called "La Source," holding a stone pitcher on her shoulder.
Could it possibly be that he was shocked at seeing her sitting there?
She also remembered how Little Billee had always been silent whenever she alluded to her posing for the "altogether," as she called it, and had sometimes looked pained and always very grave.
PS (as I am sure you may know) The Trilby hat is named after the Trilby in the book.
PPS I feel I must add this tragic footnote from the Wikipedia page:
The model for the painting was the young daughter of Ingres' concierge. In his Confessions of a Young Man, Irish novelist George Moore wrote, with relation to the morality of artistic production, "What care I that the virtue of some sixteen-year-old maid was the price for Ingres' La Source? That the model died of drink and disease in the hospital is nothing when compared with the essential that I should have La Source, that exquisite dream of innocence."