Not sure I get what the result should be.
Could you simply define only the filled voxels as as primitives into an OptiX geometry acceleration structure (GAS), maybe per object if required under an Instance AS (IAS), let’s say by defining the surface between two voxel volumes as two triangles, and then shoot rays at that to find the nearest intersection?
That way both BVH traversal and triangle intersection would fully run in hardware in RTX boards.
See previous discussions about voxels here:
https://forums.developer.nvidia.com/t/best-practice-voxels-use-triangles-or-custom-geometry-for-calculations-not-rendering/115230
https://forums.developer.nvidia.com/t/using-rtx-acceleration-for-voxel-tracing/74007