KB, as someone with a similar problem on the other hand I can sympathize with your situation. I had a great deal of difficulty with the turn signals after running my thumb through a power saw (turns out that isn't a good way to trim your nails), since the thumb is now shorter and has restricted movement. My answer was to add a simple extension to allow my shorter thumb to reach the button without the jerkies involved in moving my entire throttle hand. Perhaps something similar could be done to your horn button. I didn't take the housing apart on the clutch side to look at the mounting of the horn buttom, but assuming it is similar in construction to the other switches I'm thinking a simple remove, flip, and reinstall isn't going to work due to the way the mounting bracket is made.
Jerry
btw, my homemade extension was bent out of a piece of heavy duty clothes hanger wire.