You can gain access to the "Map Texture", which is called the ScanEnvironment shader, from the OpenBE.xcodeproj within the Bridget App. Look under OpenBE.xcodeproj > OpenBE > Shaders > ScanEnvironment and ScanBeam folders. Since they are shading tools, they will be in the .vsh and .fsh file formats.
As for only being able to scan the floor, you might want to look into the ScanEnvironmentShader.h and .mm files for hints. There currently is no turnkey solution to this, and, from my understanding of the Bridge Engine, this may be quite difficult to achieve.
I think what you'll want to do is 'detect' the floor and not just only map the floor, as mapping/scanning only the floor of an environment would be very difficult and probably not useful for whatever you are trying to achieve.
I believe that the floor of an environment scanned is going to be at y=0, which is a good start at trying to detect the floor plane.
Could you explain a little more about what you are trying to do with the Bridge Engine? This will help determine the best advice to give you.