i think his question has less to do with how your system will render the information about what's behind the wall, and more to do with how your system will acquire that information in the first place
- high-res satellite and airplane imaging of the globe
- public geotagged photos, including compass bearing
- Google Street View road images
- public security cameras on public streets
All of this data can be integrated using software like Microsoft's PhotoSynth, to stitch together a gigantic 3D model of the real globe.
Then, people with Voxel Vision will be able to look around at streets and landmarks that are interesting but physically occluded.
I think he must not have gotten the sonar parts in his half bat bits.