Hi ! I finally have some time to give some news.
For your idea reminiz, the problem is that those are for white objets only. This would need a special kind of printing like decals with white available in the printer also. I know some factories that do exactly that, but it's expensive.
Sfbp, I understand your fear, but it doesn't seems to be a problem. I haven't really manipulated them enough to be sure of it.
I'll go back to it, do some more and make sure it stays ok over time & handling, and if everything's fine, i'll post some detailed steps on how to get them like this, along with a photoshop template if anybody's interested.