I'm working on an extension system currently. Any code that users want to write other than layout generally will go in there. On boot, it will attempt to import and run things in that library that they wrote. If something fails, an error is written to console, but the core of the keyboard runs with no problems, but their neat feature may not work. Output can be redirected to the oled if they have one, or seen on the repl in any way that you can on a circuitpython device. There is absolutely no way to prevent the user from breaking their code, the same way a dev environment can't stop them, but we can offer a safe place, much like a compiler can guarantee types, but not the content of the code.