What really got me thinking about this was a picture of some sort of arabic hand tattooing. Sorry I don't know if it has a more official name. I've attached a picture of the kind of thing I'm talking about.
I'm not suggesting knife play on the hands per se, because of course that would be displayed rather prominantly in public. I think the best spot might even be on the back, perhaps. But the thought of being able to do some sort of real detail work with a knife, something artistic like this, the thought intrigues me. The only problem with the back is it makes it pretty hard for the sub to see what you did, lol.
Personally I don't know that I'd be the one to do it because i'm not that artistic when it comes to drawing. But the thought holds intrigue for me.