On linux, it\'s possible to create a tun interface using a tun driver which provides a \"network interface psuedo-device\" that can be treated as a regular network interface
In the Singularity project, Microsoft research communicates with the singularity VM through a "loopback" adapter. Maybe that'd help?
Running it is easy so it may be something fun to do anyway. :)
http://research.microsoft.com/os/Singularity/