If you want the texture on the cube to be flipped across the x axis then just change the vertex uv's coords of the entity which your texturing.
On a cube for example:
[EDIT] Thinking about it - this is probably not what your after. I'd check out the full screen blur routines in the archives - they will help you in adjusting the uv's ( although the U they'll still need flipped ) and scaling the cube to fit the screen correctly.
On a cube for example:
cube = createcube() FlipU( cube ) function FlipU( entity ) s = getsurface( entity , 1 ) for v = 0 to countvertices(s)-1 if vertexu( s, v ) = 1 - vertexu( s, v ) next end function
[EDIT] Thinking about it - this is probably not what your after. I'd check out the full screen blur routines in the archives - they will help you in adjusting the uv's ( although the U they'll still need flipped ) and scaling the cube to fit the screen correctly.