I think the Yeti and the MamJama should keep you warm down to the 20's. I would not bring a heavy blanket or a yoga mat if it were me...just to much weight. I would make sure you have Nalgean bottle or something to boil water and throw in the hammock with you. Works awesome. Also one thing I do to keep my feet warm is get some To Warmers.
I use these http://www.amazon.com/HeatMax-Toasti...xp_grid_pt_0_0
They actually work awesome and don't weigh much at all!!! Stick right to your socks and your toes stay warm all night. I got mine at Sam's club fyi
I use a trash compactor bag inside my backpack for all my sleeping gear and cloths. stick them all in and fold over the top. This helps make sure your gear is not wet when you get there.