At last we have something that looks like a pixels burnt in to toast.
Method
Stamp the pixels in a matrix pattern, using the end of a pencil (rubber) in to the bread.
Then toast. The mirror image come out better than front-side. Partner suggested it was not appealing (edible) but I think the mirror side is not too bad and is presentable and represents the pattern stamped.
I think this method could work. Stamp out a pattern, then toast. I am thinking, something like a teleprinter tty ? or toast-Teletype ? Where each line of dots across a row is stamped out.