For anyone looking for an advanced sparse voxel data structure, there's OpenVDB developed by DreamWorks Animation:
It's developed for film applications which naturally have a different set of requirements than realtime game engines. But as PC performance grows, the solutions developed for film rendering will become more directly adaptable for games.
And we used the same data-structures in the modder voxel editors.
So its not really an 'open question' as the article says, more just an obvious step.