OK.  Let me be a bit more specific.  The light-weight down jacket I have (North Face Summit Series) is rated at 800 fill, but comes in at under 1 lb total weight.  The pants I'd be looking at (e.g., Montbell UL inner pants) would also be 800-fill and under 1 lb.  So, I'd be looking to keep weight low and yet have some flexibility of use (other than sleeping). 

I already have an over-sized down bag that unzips completely.  I don't' think it would be much of improvement to go to a down blanket.  (I also wear head gear, even with the down bag, when its cold ... I'm not fond of having my head stuffed inside the bag.) ... and yes, I'd have to also cover hands and feet (down booties, maybe).

So, given these parameters, can you think of any inherent reason that this would not work to temps to about 20 degrees?  One of my concerns is the radiant heat produced by my body being sufficient to maintain warmth when my extremities are separated.

dd