That’s actually what I’m toying around with (vacation aside). Encoding lighting information in a layered reflective shadow map (recent paper) for individual lights, and then encoding the standard geometry/color information into a set of 3d texture blocks, which being coherent in memory are a lot faster than sparse octrees, and reducing then memory footprint by storing the unlit voxel information into a directed acylic graph. Combined with some more recent hardware features allowing 3d textures to use null pointers for, in this case, empty voxels the resulting memory hit should be very low, while the structure should be very fast.
It’s half an idea with pieces of broken code right now though. I still haven’t put out my Pixar ambient hack demo, which is another huge time saver (no need for double light bounce to still get information). And that’s mostly working. Maybe someone with a lot more time than me at Epic will read this and actually accomplish something though.