SFC can use cubed, infact just about any shaped models for use as a background
Yes cubed works. I used a cube model to add in Hubble pics as a background. You need high resolution pics in SFC3 as the engine magnifies any image files big time, so some low resolution images won't work too well.
What I actually used was just 8 seperate squares in the 8 directions, they were not conected but they act the same as a cube. If I remember you can compensate some for the extreme magnification of the SFC3 engine by moving out the square further from center.