I got smooth shaded voxel ambient occlusion working.  I've implemented it in such a way as to have very little performance impact on the GLSL ray marching shader I wrote and this is all done in one render pass.  I've also managed to still fit each voxel's data into a 32-bit integer which includes 16-bit RGB color (565), ambient occlusion weight value and a respective ambient occlusion weight centeroid.

There are some artifacts in the smooth ambient occlusion which I may be able to fix.

Flat ambient occlusion:

Smooth ambient occlusion: