<p>I want to put my master pages in a central library so I can use them in several projects without having the maintanance nightmare.
Everything is refactored in a generic and central way, they are all in a "shared" namespace.
But if put them in a seperate project, I can't reference them</p>

I don't believe you can put that actual .master pages in an external library. You can place your own customer MasterPage-derived class in the library, and have your client apps derive from that, but I don't think that's what your intent is, since you're tagged with asp.net-mvc (implying very narrow views without much logic, and probably no code-behinds).
As far as I'm aware, there isn't a way to reference a master (nor, for that matter, an ASPX or ASCX) from an external library. Wish I had better news for you. (And I actually hope I'm wrong; I hope someone else here has figured out a technique for this, as I'd like to use that kind of thing myself on occasion.)