The answers all look the same: it’s not possible, you need to wrap it yourself and not use UnityEngine.Input.
Sadly, the wishlist-link from the first post cannot be found anymore. Since we reached Unity 5 already, has anything about this changed?
On the other note I need to describe my real needs: In my project there are several different types of pointer devices. We have an absolute pointing device (like a graphics-tablet), we have a Gaze-Tracker and we have relative pointing devices. Only one of those will/can be enabled at a time, but they should behave the same: giving a current pointer position just like a mouse.
I also want to mention: I would also create a native c++ plugin if that would be neccessary to do the trick and on the other hand restrict the target platform. It’s just a fact: having to use multiple different inputs non-unified to UnityEngine.Input simply seems like the wrong way to go.
Any good news on this would be greatly appreciated, maybe som Unity-Devs would care to make a statement?