apparently it has been done Jeremy, i've read a few references to guys that have laminated the keel out of plywood. No idea how durable this will be regarding water ingress. If I have to go this way i'll be epoxying a hardwood sole onto it and encapsulating the whole shebang pretty thoroughly. some guys talk about simply painting it but I put a lot more confidence in epoxy and some kind of fabric ( glass, dynel, etc) to seal it up tight.
have fun.