Both PSTricks and TikZ/PGF support polygon options
(repsectively transpalpha and fill opacity)
that have the effect of making the polygon appear transparent. Objects
behind (drawn first) are visible with color subdued and tinted by the
polygon in front. The hidden surface algorithm of sketch works
well with such transparent polygons.
Note however that cull=false must be given for rear-facing polygons that need to be visible when positioned behind other transparent surfaces.