UV problem with billboard particles

I don’t understand how to assign a UV map to billboard particles. I initially thought that I would make a plane and the billboard particle system would then use that plane as a particle but this is not the case.

I have a simple cube as my emitter but billboards are planes generated by some script. How am I supposed to assign those nonexistant planes UVs when my emitter is a cube? If I make a (UV mapped) plane the emitter the UVs work fine but I don’t understand how I’m supposed to do this with any other shapes.

I’m going to bump this since I would still like an answer. I’ll rephrase my question.

My emitter is a cube. It emits planes and those planes have a texture assigned to them. I can see the texture on the plane when I render but the texture mapping is off, the texture is cut off. Is there a method to UV unwrap those generated planes? Here’s an example Blend.

Billboard Particle System Example.blend (120 KB)
On layer 2 is the cube emitter and particles with incorrect texture mapping. On layer 3 is a plane emitter and particles that have correct texture mapping. I want to use a cube as an emitter but with the texture mapping like with the plane emitter.