Just downloaded Unity to quickly check that it's possible, which ended up being pretty easy to enable.

I've documented how to add Lisp scripting to an existing project at:

https://sharpscript.net/lisp/unity

Here's a youtube demo of scripting game objects at runtime with an in-game REPL at:

https://www.youtube.com/watch?v=WmzI-5L0hJk