Why face up? I believe that to get a reflection free IR, you need to in effect remove walls and ceiling from the recording (hence why the mic should be placed on the floor). If the cab were face up (presuming the mic is suspended in the space above it), this would be no different from placing the cab against a wall indoors.
Because I can place sound absorbers and deflection behind the cab such that no boundary reflections will reach the mic within the desired time window.
This is impossible to do in my live room. 24x34x13.