I don't know how to waterproof a seat but you can buy a rain cover made for seats suitable to ride on. I bought one from Mustang Seats and it works good. It covers the rider and passenger seat and the drivers backrest. Check it out if you're interested: http://www.mustangseats.com/Categori...ers&reset=true
I like to use HD leather dressing and rub it into the seams. I have used it on the bags of my 05RKC and Corbin seat for years and it kept them like new. I now use it on my C & C and it keeps the leather nice, I also use it on my gloves to keep them soft and dry,
....... i'm reluctant to mention it, but my substantive posterior provides all the protection from rain that my saddle ever gets.
.... except for the garage, of course ....