I decided to take advantage of the REI 20% coupon and stumbled across the Sierra Designs Mobile Mummy. It's a Mummy Bag that has a top center zip, arm holes and a legs hole so you can wear it around camp (sorta like a Snivler.)
I went ahead and bought it and we'll see how it does.
I'm hoping that the top center zip will make it easier to use in a Hammock. I'm hoping the ability to wear it sorta like a jacket will ease the transitions between minimal insulation while moving and tons of insulation while stationary. I'm hoping that the ability to stick my arms out will help me retain warmth when I have to reach out and adjust anything on my underquilt.
Please no "you should use a top-quilt" comments in this particular thread.