I think the beaver is a bad call, you are going to have to run a lot of air pressure to not flat tire.
Pemby doesn't really get muddy or greasy in the rain, if anything the grip gets better. I would stick with your original tires. An Icon or Racing Ralf would be the way to go for a rear tire.
I road it way back in 2010 in the pouring rain with CrossMarks and they worked great.
Thanks for the suggestion, I've removed the Beaver (damn it took me SO long to finally mount tubeless too!) and put on the Ralph…