You can use the Cube() constructor "tile6" param, which will allow you to assign a single bitmap material that has been set up with a single texture with a 2x3 grid. Your face textures would have to be pre-scaled to fit into those 6 slots.
Other than that, you must set up your own Mesh with 6 separate SubGeometry's so that you can apply 6 separate materials to separate SubMeshes. -Dave
