Short answer: engine only. It's a shader thing, so I wouldn't expect any standard 3d package shaders to support it. We use the alpha in the diffuse in different ways, mostly for alpha but sometimes as a mask for a skin shader.
MMmmm i dont know about that, i know in max's material system its easy to do, but i dont think i've ever come accross a .fx shader for max that lets you use specular from your diffuse alpha channel, i'm pretty sure you'de have to have something custom to accomplish this.