In August 2010 Adrian Glew, a Tate archivist, announced evidence for a real-life Quasimodo, a "humpbacked [stone] carver" who worked at Notre Dame during the 1820s. The evidence is contained in the memoirs of Henry Sibson, a 19th-century British sculptor who worked at Notre Dame at around the same time Hugo wrote the novel.From a 2010 article in The Telegraph, via Wikipedia.
Sibson describes a humpbacked stonemason working there: "He was the carver under the Government sculptor whose name I forget as I had no interaction with him, all that I know is that he was humpbacked and he did not like to mix with carvers."
Because Victor Hugo had close links with the restoration of the cathedral it is likely he was aware of the unnamed "humpbacked carver" nicknamed "Le Bossu", who oversaw "Monsieur Trajin".
Adrian Glew also uncovered that both the hunchback and Hugo were living in the same town of Saint Germain-des-Pres in 1833, and in early drafts of Les Misérables, Hugo named the main character "Jean Trajin" (the same name as the unnamed hunchback carver's employee), but later changed it to "Jean Valjean".