Some speculation here. I've not tried this idea out yet.
I've been doing a little reading up on Sparse Voxel Octrees lately, and an idea in Samuli Laine's papers particularly interested me. In these papers, "contours" are used to add shape to the basic octree voxels to enhance detail without extra subdivision.
I quite like this idea. I've been thinking of ways to try to extend the contour idea.
How about using a marching-cubes type algorithm to provide some more detailed geometry at the node or leaf level, based on the existence of non-existence of neighbouring octree cells?
You could quickly construct a bit mask based on the existence of neighbouring cells, use that to look up in a table providing several specific pieces of geometry for each case. This could add some extra detail and shape to each octree voxel.