I know someone that had an extension welded on before. I should talk to him and see how it held up to his riding. One problem that you'd run in to is that you'd need a longer compression bolt, or have a new compression system put in with the extension.
While I think companies offering a variety of lengths would be a nice solution, it isn't feasible from their perspective. I just don't get why they don't offer a long length....who doesn' have access to a hacksaw?!