I am a weather weenie, I admit. However, I'm wondering why a tarp appropriately staked and protected from wind as much as possible via site selection, and a hammock sock over the hammock/underquilt wouldn't provide the needed protection? The hammock sock I'm thinking of would have a bottom made of dwr/cuben/sil (pick one) and the top would have something breathable (1.1 ripstop or even netting). Any snow that gets under the tarp would be caught by the sock. The wind would be blocked. That just leaves limbs falling (a problem with site selection) or the guylines pulling out or the tarp ripping, which would be an issue with tents, too.
I know from talking to Nest that he feels that the underquilt and sock are why he was able to carry his hammock all the way on his thru hike this year. He said he and Cannibal were the only ones who seemed to be able to deal with the cold on the trail because of their underquilts. He can step in and add to this when he gets back to the forum.
Bad spellers of the world Untie!