Why can't you use additional UV channels to achieve this ? With the right shader it would appear in the viewport. standard composite mat, with pow set to map channel 2 (no tiling) and planar map set to channel 2 (pow is just an 32 bit bmp). Though you have to be careful as it would appear on the backside too :)
Using a "composite map" in the diffuse slot can get the job done. http://www.cgrats.com/apply-decals-using-a-composite-map.html But what you probably want is something more like this: [ame="http://www.youtube.com/watch?v=ge59E3n7gwo"]turboTools - Decal Projector ( 3DS Max) - YouTube[/ame] which I don't think Matt released…