Multiple mice in Unity

I know multiple mice interfaces can be implemented in Flash through a C# workaround with the Microsoft WPF. Can the same be done with Unity?

No. Unity only has support for a single pointing device. It also only supports one keyboard, and I believe up to 4 joysticks/gamepads, though I could be mistaken.

The best you could try is writing a C++ plugin to handle it, which requires Unity Pro, and a fairly extensive understanding of the Unity subsystem, though even then I'm not sure it would work.

Of course it is possible, you have to implement custom C DLL for DLLImport, or a custom C# DLL.
And somebody did this already: Multiple Mice Input in Unity | Alastair Aitchison

i don't know anything about the workaround that you are talking about but you can write a service/app to handle it and then communicate with unity using sockets.

Microsoft has an SDK for multiple mouse support, available here ...

I'm not sure if dllimport would be sufficient to call this from Unity (but I'd love to hear from you if you try it :-)