I'm also looking into this sort of setup, I think a bunch of people were about 20 years ago, and there is a new surge of interested based on AC 2013. I think the 18 may be a bit of a pig for such a project. There is at least one video on youtube of an 18 setup. No idea how it worked out in practice. The biggest concerns I've come across in my own investigation has been power to weight ratio... (use a bigger sail?) and that in addition to the daggerboard foils, you'll need to design/ setup a stronger rudder mount/ rudder system. It can happen!
Fleet 259, Central Coast CA