Hmmm...
I think I get what your after, converting a 3d scene into a cube map?
Off the top of my head could you not use something like HDR shop to do it?
That converts spherical, panoramic and cube maps maps such as HDR images into cube maps (or spherical or panoramics) pertty much in the touch of a button? You just need an input image to do it.
What you might have to do is have your environment make a sphere, up the reflectivity to 1 and just render the sphere, nothing else but make sure that everything is lighting/reflecting the sphere but its primary visibility is set to off, then use HDR shop to convert it?
It might be worth getting HDR shop (its free) and some spherical or panoramic HDR (or LDR dosent really matter for a test) images (free also from the same site if I remember correctly) and then seeing if HDR shop does what you want, as I might have got it wrong with what your after.
Hope its of some help.
Cheers
Steve
"No pressure, no diamonds" Thomas Carlyle