IMHO, libraries like this should always have an easy "out", where the default is strongly typed, but the user of the library can explicitly escape the known behavior if need be.

As a (albeit poor example), I once inherited a click tracking system that uses Java Jersey. The standard response object has a .redirect(URI uri) method. Unfortunately, many "URI"s we receive from the world are not actually value, and the java URI object refuses to build URI objects from the string. Fortunately, one can simply call .setHeader(String, String) and be done with it.