I would tread carefully. I have seen several attempts to do this fail by having more strictness than buy-in.
We take buy-in very seriously and the pattern library itself is being structured by applying a UCD process (the users being our analysts who do the GUI specification and will use the patterns). We are especially looking at how to provide access to the pattern library to adapt it in the best possible way to the requirement engineering process and make it accessible from inside our prototyping tools.
Concerning strictness: Do you think a 'prescriptive' pattern library is stricter than a style guide that has to be followed?