If the width of a trailed weedwiper is too much, there's another gadget you could use instead. It attaches to the front rack of the quad and sticks out the front, low down, like the "bull-bars" you sometimes see on pickup trucks. It uses a wick to apply herbicide to the weeds in front of the quad as you drive over them.
However, one advantage of the trailed weedwiper is that you can offset it to left or right, which might be safer for working on a river bank.