Joos is a *short form*, not a diminutive - if it were, it would have been something along the lines of Joosje, Jooske, etc. [noted -ed]Also, apart from what is already listed in the database entry, Joos is also a variant form of Joas in The Netherlands.
Hmm. Since I tend to pronounce things the way they are spelled phonentically, I'd pronounce this name like the word "Jews". I wouldn't use this name. Not that I've anything against Jews, it's just that I wouldn't use this name.