On plans for the improved stacking materials mentioned in the roadmap, that would be one for the devs to answer :)
Not 100% sure what effect you are trying to achieve, a render or screengrab may help. If you want to avoid the complexities of UV mapping, you could clone your object, select just the polys you want the material on, and delete the rest; then apply a material with Opacity to that new object (with your label / decal), and use any desired mapping in the Material tag for that object. In my tests, I didn't even need to move the polys so that they didn't overlap, but your mileage may vary.
I used a Flat projection in the material tag, and scaled and rotated that to align things (no tiling on the label material), though could use a Frontal projection too, or any other suitable form. This approach doesn't use the full UV unwrapping (though you could still do that, separating out the polys for the area that needs the label / decal would make it easier to get a good UV mapping for it.)