Could you combine this with virtual texturing/megatexture to increase the GPU cube size...?
In case you only would want to show part of a huge volume that could work.
For showing more of a large volume, mip mapping or quadrilinear is best anyway.
I'm begining to have visions of vast voxel worlds rendered this way, with all kinds of unseen volumetric effects applied to walls ...
I'm wondering if there are artists and tools that could create the volumes needed.