Here’s my question: I am creating a font where the keycaps on my keyboard are re-illustrated. Now I notice that I can’t confirm, for example, an “Enter key” as a selection for an illustration I want to have when the Enter key is pressed. I know it doesn’t normally make sense, since the Enter key (I have a Qwertz layout) serves as a paragraph. However, I want the Enter key to not have a paragraph, but the illustration I create for the font. So how do I select an illustration to appear when the enter key (or command keys like ctrl or alt) is pressed?
How to create a Font for the special keys?
Short answer: you can’t 
Action keys like control, shift or alt are not tied to specific characters, and Fontself doesn’t support any of the hacks to do end-of-line custom glyphs.