Great idea for a puzzle game! I was skeptical on it potentially being just a gimmick until I realized that the keys can end up OUTSIDE the keyboard and you won't be able to click them anymore. Building the puzzles around that and trying to rig it so that your keys are available when you need them is neat!
I would personally definitely recommend some sort of UI that tells me what the current mapping of my keys are and also mark any that are currently out of bounds. It can be somewhat confusing to keep track mentally of where the mapping has currently gone to.