Note: since version 1.3 you can simply select Opaque 6-textures as render type in your voxel definition to generate a voxel with 6 different textures (it also supports native rotation).

With version 0.20, Voxel Play includes a custom cube creator tool. This tool allows you to create a cube prefab capable of using more than 1 texture per side.

The tool will automatically pack the 6 textures into one texture atlas: