Wooooow that's actually pretty awesome. I didn't know you could do that!
Yep, as long as the artist uses a disposable tube instead of the stainless steel, I just bought one and brought it to my buddy to do it with, the ashes themselves are sterile so as long as everything else is done properly and the ink and ashes are mixed together well there's no problems. It just darkens the ink up a bit.