Ahh, the trouble with handling it this way is that (as far as I'm aware) there's no way to handle acceleration, deceleration, jump sustain, etc. It would completely change how the game handles when you change the control layout.
Man, that's a shame. I guess I'll just have to go with arrow key movement, and abandon the control options idea
Thanks for the suggestion though! There might be a way to pull it off with this method, but it would likely involve scripting out huge chunks of Construct's platform behaviour from scratch. I can't believe this option isn't included as standard.