I wouldn't advise trying to make an H out of two 1s in itself, you might get a bit of attention from the plod. I'm guessing the 1s will have to be quite close to achieve what you want, so you're asking for a pull before you even stick a black dot on. But anyway, that's your choice.
The little plastic head that covers a screw - can't you glue that on without the screw? And maybe stick another black one over another letter, roughly in the same place on the other half of the plate, just so it looks a little less obvious.
I see plates with one random screw cover, or one black and one white, and it just looks so obvious once you get within a few feet. I'm not sure there's a proper covert way of doing it!
