Hey Mark, you probably don't remember me but I helped you out with a few vagcom tweaks a while back. Anyways, you should give Ed @ Induktion Motorsports a call. They're located down by BWI and specialize in Audi/VW/BMW/Mini/Porsche. My only experience with Induktion is that I recently had an alignment done there, but after chatting with Ed I'm sure carbon cleaning will be a cinch for them.
New German Performance is another option but they're a little further, in Aberdeen. I have no experience with them but they're a large VW/Audi focused shop.
I'll be doing my own carbon cleaning within the next few months, just have to put together my walnut blasting setup!