Unconvential Sleep Systems

I haven't used them, but feathered friends has some oddities:
http://featheredfriends.com/Picasso/Bags/Specialty.html
The Vireo is interesting,
" It has no zipper, no hood, a variable down fill and weighs only 16 ounces. A variable down fill means that the lower part of the bag has more down fill (rated to 25) and the upper section has less fill (a 45 rating). It is designed to be used with the Volant Jacket (or Icefall Parka or Frontpoint Parka) for full protection."

I knew someone who used a half quilt for the lower half of the body and wore their down jacket to bed at night.

I have used a down jacket, down pants, quallofil booties, contact gloves, balaclava inside a -5C down bag to stretch its comfort level to about -10C. The big advantage of this approach is that you can keep wearing the down pants and jacket when you get up, which is typically the coldest part of the day anyways, and keep that body warmth with you as you break camp. The downside of this is that if the weather is bad while you are breaking camp, the down jacket and pants might get damp or wet, meaning a cold sleep that evening…! A synthetic Wallcreeper overbag is similar, and worth exploring…as is a Tyvek painter's suit or trash bag vapour barrier for below freezing—I carry one, but seldom use it, but I have it, just in case!

I have used half bags with jackets though the years, typically this set up will weigh more then a straightforward sleeping bag.
My arguement is if you are going to carry an insulated jacket then this way makes sense but it takes a pretty good jacket to bivy at 20 degees F., pretty fair gloves and auxillary head wear where it only requires a moderate quality bag. There is a completely different level of insulation needed for at rest versus light activity in jackets.
I do like the flexability of sleeping this way especially if you need to be tied in [to a safety system], also it is a great way to sleep in a hammock but my experience runs to an old Hatteras rope hammock not our more modern ones.
A longtime ago down pants could be rezipped into a half bag configuration, they usually had poor draft flaps but since you jammed your entire lower trunk into your pack and pulled the snow collar up around your waist to bivy.
A moderate 15 degree bag like a Marmot Helium weighs slightly less then 1#15oz. Wild Things Half Bag weighs 1#13oz. This would leave you just 2 oz. to come up with a jacket, gloves, and headwear of a pretty decent thermal value. It would be very easy to exceed 4#, even with a cost is no object approach you would still be hanging in over 3# with a wear everything ya brung approach that is just too marginal in the alpine environments I hang around.
There are and were a few people who like to roll the dice, calculating their energy needs, thermal values and pack accordingly light. [The Mark Twight plan]
I know of no projects that failed because of an additional pound or two of equipment but I do know of many that have because of being cold.
I like my half bag setup a great deal but I don't argue the benefits on a weight only basis.