Jack,

I think physics might not allow a perfect solution. To spray 100', your herbicide would have to be in the form of a fluid slug, rather than a mist. When the slug hits your duckweed, a large portion would be wasted into the pond rather than spread across the target plant.

I have an inexpensive 15 gallon sprayer that I use out of the bed of my pickup. I just attach the clips to a deep cycle 12V battery. I always run out of liquid long before I run out of electrical charge.

That set-up should work fine in a canoe. It should spray a little farther than your backpack sprayer and your canoe will be much more stable with you spraying while seated. (You can distribute the weight along the length of the canoe: battery, sprayer, you.)

Good luck getting your duckweed knocked back to the correct area!