Rather than mock the file system interface, I'd traditionally opt to mock the Filesystem itself. This is a relatively easy task on any serious OS. You can build a userspace filesystem very easily with Mono.Fuse, a C# implementation of FUSE, Filesystems in USErspace. I'm not sure what if any porting would be required to get Dokan, FUSE for Windows, but FUSE works fine on Linux, OSX, and Solaris.
