Decals don’t work on transparent objects afaik (well transparency is kind of lacking in ue4 in general if you ask me) but you don’t need a decal to render a texture on an object in the first place! 
you can use a translucent material on the curved mesh with masking etc. then you could pass parameters to move the “target boxes” around but that’s probably going to be a bit complex to implement.
But that approach won’t get you very far anyway, especially if you want dynamic text stuff on the screen (which cannot be done with a material).
You can wait for 4.6 or use one of the html ui plugins to render on the mesh (although I do not know if all of them have proper transparency support).