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.