The reason you've not had a direct answer is because its a complete non-issue, something you'd have quickly realised in the first hour of driving a narrowboat.
I am lefthanded and our boat is assymetrical, in that its a semi-trad stern with a built-in bench seat on the left, a wide enclosed area to the right and a door on the right to the cabin. Its actually pretty comfortable, to sit on a stool on the right and hold the tiller with the left hand. You see, throttle adjustments don't happen very often - you're not planning on slowing down through bridges or stuff like that?? For passing moving boats, it really doesn't matter if you're on the left or right, you can judge where the boat is. Just like you know where the kerb is when you're driving a car.
My girlfriend is also lefthanded but drives stood up and can do it both ways.