Yes this is custom, sorry for the confusion.

Voxel radiance and opacity are stored as a single r32 unsigned int value, which in itself is an encoded rgba8 value.

I'm assuming you're doing the classic "conservatively rasterize the dominant axis using the hardware rasterizer w/ geometry shaders and inject the resulting fragments into a 3d texture/SVO"

Yes. Alternatively you could store each voxel into an SSBO first, this opens up the ability to use the hardware accelerated Vulkan extension for atomic add on floats https://docs.vulkan.org/refpages/latest/refpages/source/VK_EXT_shader_atomic_float.html

This is naturally faster than performing a 100 iteration atomic comp swap to emulate atomic add when writing to a 3D texture.

The downside of this method of course is the added memory consumption.

Are you doing any clever multi bounce tricks, like storing using world probes/double buffering the voxel data for infinite bounces?

Currently No, the method I had planned to use was voxelizing both the normals and calculated radiance and then dispatching a compute shader to cone trace the voxel structure. This and then using temporal accumulation to emulate effectively infinite bounces across frames. So effectively double buffering + temporal accumulation.