I am going to rebuild my tyvek bivy with more room so I can get my pad and an over quilt into the bivy.
This would be mostly for tent sleeping.
I originally wanted to build an XP quilt and velcro it to the top side of a tyvek bivy in a few spots, then it would just wrap over my #2 MB bag.
Then I started thinking, why not just build a bivy with the insul attached, IE WP breathable material outside, insulation then an inner of .5 oz silk or something like that.
Have a detacable hood via velcro or a zip that you could flip from one side to the other.
IE you could use it alone with no bag as a quilt on top, flip it and have the insul underneath for really hot nights.
You could also do the same with the bag inside, IE if you need extra warmth, insul on top, just need the water protection, insul on bottom.
I think I might have to try this one out.
The only downdside I can see is the entry opening being on the opposite side of the sleeping bag with one setup.
It would not save much weight, but it seems like it would be more convenient and one less thing to carry.