I haven't... but that would be a cool idea...
I would just be concerned that if such a service existed, people would see a lot of the same questions over and over again, and someone would come up with a "cheat sheet" of answers...
I know its's more work, but coming up with your own questions and scenarios means that its more tailored to your needs... and also it would force your candidates to think on their feet. We did that in our last round of interviews... we also made it clear that we were looking more into the thought process behind the answer... but we weren't looking specifically for an "implementation specialist" but more of person that could do a variety of analytic tasks (cause we are a small team, we don't have dedicated positions)