DentonDoc wrote on Oct 25^th, 2011 at 12:54pm:

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

I think you should be fine.  Doesn't the body shut down the extremeties when the core gets too cold?  So long as you have enough insulation you should be fine.  The link from Kingfisher is essentially what you're planning to do in a onesie.

This is definitely something to test on a 1-2 nighter before going for a week.  Fortunately it's the right time of year for the test.

I suspect that the added weight/bulk of replacing the sleeping bag will be a net zero result.  The big bonus to look for being comfort.