I have seen lots of boat trailers with the licence plate mounted on the right. Cant say what the law may be in your neck of the woods but in BC it says they only need to be:
- securely fastened in a horizontal position to the vehicle
- must be kept entirely unobstructed and free from dirt or foreign material, so that the numbers and letters on it may be plainly seen and read at all times and so that the numbers and letters may be accurately photographed using a speed monitoring device or traffic light
- Also need to have a light
on them at night.