Here is an interesting post from Chris about dynamic transformations within the OSB.
It looks great, but think about this:
You loose the strong typing of the messages the proxy can receive (as described in the blog itself). This may lead to faults that are difficult to trace because the service contract is in fact far more coarse grained.
How about maintenance? I like the approach that for a different major version of a service with different XML data, use a different proxy. This way services of different versions can live together and services can become obsolete when clients are no longer using the old service. If you try to put all versions of the service in one proxy service it can soonly become unmanageable.
But it can still be useful to have dynamic transformations.