A lot of people rig downwind sails
for kayaks, some use kites, some use basically a piece of fabric
stretched between two poles and set up to catch the wind
if you want to get complicated, Hobie makes a kayak-like boat with two outriggers, amas might be the word in this case, to make a trimaran
and it uses a boomless mainsail Hobie Mirage Adventure Island
It still uses a daggerboard, but I've considered doing something similar with my kayaks and I don't believe that a leeboard type system would be too hard to rig up with any normal kayak.