If your dermals are where say, a regular nape piercing would be, then you might have a problem. It doesn't necessarily move around a lot, but it does have the potential to slip one way or the other if your leg moves funny. I'd almost be more worried about the pressure on them from the sling in general holding up the legs causing more irritation. And if they aren't completely healed yet, I wouldn't use anything like this near them for fear of them falling out. All that being said about use behind the neck, you can also slide the sling down and use it behind your shoulders rather than your neck. At least with the ones that are more like a strap for that part. The ones shaped like pillows can't do that, although they might hold in place better on the neck I think.
Wow I started rambling. I hope that didn't just get a whole lot more confusing.