The breath bib is the way to go.
Maybe some velcro tabs every 8 inches sewn to the hammock and fly, then a pieces of silnylon cut to the shape.
You might want to use something other than silnylon for the breath bib. Sil is so slippery that condensed/frozen breath moisture will slide down with any movement. The frost from your breath would end up on your face or neck! Try fleece instead - it gives the ice something on which to grip.