I tried this three character technique yesterday before reading your posts, I have gotten it to work that way. Got the desired space to the right of the box with a tracking of 200 in the drop cap character style. My concern is how it will work in an editorial environment where the proof readers are sure to remove the spaces. But if they are informed on this then maybe we can get it to work. Below is a picture of how it looks.

I also tried using Whitney with black squares, which would work perfectly if it wasn't for the fact that Whitney Index black squares doesn't support Icelandic characters – so that is a no go for me.

Thank you so much for your help, this has really been informing.