Nos livros de Harry Potter, o que os bruxos vestem sob suas vestes?

Nos livros, basicamente não há menção de bruxos vestindo algo sob suas vestes que não sejam cuecas (ou em casos extremos nada mesmo).

Ordem da Fenix

Snape was hanging upside-down in the air, his robes falling over his head to reveal skinny, pallid legs and a pair of greying underpants.

Prisioneiro de Azkaban

Harry took off his black school robes, removed his wand from his pocket, and stuck it inside the T-shirt he was going to wear under his Quidditch robes

Relíquias da Morte (editado por mim por uma questão de concisão)

She led them down a side street, then into the shelter of a shadowy alleyway. ‘When you say you’ve got the Cloak, and clothes …’ said Harry, frowning at Hermione, who was carrying nothing except her small beaded handbag, in which she was now rummaging. ‘Yes, they’re here,’ said Hermione, and to Harry and Ron’s utter astonishment, she pulled out a pair of jeans, a sweatshirt, some maroon socks and, finally, the silvery Invisibility Cloak.

Ron, hurry up and change …’....‘When did you do all this?’ Harry asked, as Ron stripped off his robes....‘You’re amazing, you are,’ said Ron, handing her his bundled-up robes....

[observe a falta de menção dele cobrindo seu pequeno bruxo enquanto ele mudava]

cálice de Fogo

Harry, Ron and Hermione joined it, right behind a pair of men who were having a heated argument. One of them was a very old wizard who was wearing a long flowery nightgown. The other was clearly a Ministry wizard; he was holding out a pair of pinstriped trousers and almost crying with exasperation. ‘Just put them on, Archie, there’s a good chap, you can’t walk around like that, the Muggle on the gate’s already getting suspicious –’ ‘I bought this in a Muggle shop,’ said the old wizard stubbornly. ‘Muggles wear them.’ ‘Muggle women wear them, Archie, not the men, they wear these,’ said the Ministry wizard, and he brandished the pinstriped trousers. ‘I’m not putting them on,’ said old Archie in indignation. ‘I like a healthy breeze round my privates, thanks.’

e

All their teammates but Angelina were already in the changing room when they entered. [...] ‘Shut up,’ said Ron, stony-faced, pulling on his own team robes for the first time. They fitted him well considering they had been Oliver Wood’s, who was rather broader in the shoulder.

[Novamente, note que ele não sentiu a necessidade de encobrir, apesar de Alicia Spinnet estar na sala]

ATUALIZAÇÃO: Foi indicado para mim que na versão francesa dos romances, Ron está usando um "pantalon" (por exemplo, calças plissadas) sob o manto:

ses chaussures et le bas de son pantalon - _{^{Harry Potter a L'ecole Des Sorciers}}

Há uma menção canônica dos vínculos da escola de Hogwarts, no Wonderbook: Book of Spells

If you were brought up in a wizarding family with House-Elves, then you may feel that using your magic to clean up after yourself is in some way beneath you. But rest assured, the first time you need to get a Stinksap stain out of a school tie without your House-Elf there to help you, you will be grateful you learnt the Scouring Charm.
_{(Wonderbook: Book of Spells - Chapter 3)}

O conteúdo de Wonderbook: Book of Spells foi escrito por JK Rowling.

Wonderbook: Book of Spells is an enchanted book that brings spells to life around you, and includes new writing from J.K. Rowling, such as spell descriptions and stories from the wizarding world.
_{(jkrowling.com)}

É verdade que não especifica Hogwarts, e é tecnicamente possível que Miranda Goshawk estivesse escrevendo para uma das outras dez escolas de magia. Também é possível que a norma do vestuário tenha mudado nos últimos cem anos desde o livro de Goshawk.

Um pouco necro, mas eu o leio apenas como roupas íntimas, bem como em torno da ponta da poção polissuco na câmara secreta, eles especificamente não roubam as roupas de caranguejo e goyle, mas usam roupas muito grandes. Além disso, eles pedem que a privacidade seja alterada no trem no primeiro livro, se as roupas fossem semelhantes à imagem, elas poderiam ter colocado as roupas em volta de suas roupas.

Eu acho que as roupas são como roupas de monge e menos como roupões de banho em design.

Através da Seção de Pottermore sobre roupas. Implica que pelo menos alguns não usam calças sob as vestes.

Some members of the magical community go out of their way to break the clothing clause in the Statute of Secrecy. A fringe movement calling itself Fresh Air Refreshes Totally (F.A.R.T.)* insists that Muggle trousers ‘stem the magical flow at source’ and insist on wearing robes in public, in spite of repeated warnings and fines.**

* President Archie Aymslowe

** To date, they appear to have been taken as cult members by Muggles.