You can build this, at least I think this does what you want. Use a Corona Layer Shader, and a C4D native Normal Direction as the mask. Now you plug in what material you want in the front direction, what material you want in the back direction, and apply the Layer Shader to the Decal object. You could put the same base material from what you are projecting ONTO into the Base Material in the Layered Shader, but here I used a different one just so you can see which is which :) The approach here would be e.g. if you wanted a plain white back on the decal as you might find on the back of a label. Or a mirrored version of the front texture so it reads the right way.
You could also reverse the colors of the Normal Direction if you find it easier to think of the Base Material as the Decal and the Material 1 as the backside of the decal.