Saw a demo on this at a model railroad club meeting. Sand the back side of the paper item to thin it as thin as possible, then apply it to the surface wetted with water. Set it with an overcoat of thinned Elmers glue - just a light coat. You can use an artist's brush to help settle it into the low spots while wet, if required.
A similar technique is to coat the front of the paper with a layer of white glue and let it dry. Soak in water for a few minutes, then gently scrub the paper off the back. (Naturally you need a waterproof glue. Craft stores even sell a product especially for transfering magazine & newspaper photos onto craft projects; I think one brand name is "Decal-It".) You might have to use sandpaper, but fingers are usually sufficient. The thin film of glue will hold onto the top layer of paper fibers & ink, so you're left with a thin flexible image. The more paper you scrub off, the more transparent it will be, so you may need to apply it to a light surface or paint the back before applying.
Another option is to buy some blank decal film and have the signs color-copied onto the decal film. Clear decal film will give you transparent decals. White decal film will give you opaque decals, but you'll have to trim them carefully to eliminate unwanted white borders.
I don't recall "decopauge" relaxing paper enough so it would appear the, in this case, a street sign would show as being painted directly onto brick. I always thought of it as more of a plastic coating in general.