I don't have a fat bike but I had a Brooks B17 on my last touring bike, I encountered a TON of rain in about 10'000kms of cycling mostly on one long tour. I actually agree with Fietsbob on this, put a plastic bag over the saddle, a good quality one or maybe even two or three. Also do a good job of Proofide applications and for extra safety you can double an elastic band over the ends of the bags and the seat tube: beauty in ugliness!

I urge you to try and borrow a sprung saddle before buying one, I am riding on one now cause it's the only leather saddle I could find in Latin America and it's fun for a day or two but then you realise you're torquing your knees a lot more to make up for the effective seat height change every half second. Not the kind of thing you want to experiment with on a 200 buck saddle.