Hi Alex and Bernard,
From an orthographic point of view Bernard is right: according many sites the word must
always be written «Noël» with the tréma regardless of its meaning.
However, Noël that is mainly connected to the feast of Jesus birth, may also be the first name (prénom in French) of a man, as in the novel,
Noël = first name
Lescrivain = last name.
When people scream: "Noël! Noël!" it is an acclamation, if I'm not mistaken. (I have no time to re-read the novel, so if someone can validate!)
Alex, Gallica has online many versions of the novel. Personally I prefer the one from 1865 (illustrated), but there are also the 1831 one. The consultation of one of them could perhaps clarify some typographical things about the intentions of Hugo, who knows?
1) The 1865 edition:
http://gallica.bnf.fr/ark:/12148/bpt...e%20de%20Paris
2) The first edition (1831):
http://gallica.bnf.fr/ark:/12148/bpt...e%20de%20Paris