Just a thought but instead of having it zip onto a jacket, why not make the bag a little longer with elasticated draw cord at the top and integrated braces to stop it falling down. This way it could be worn with any jacket and also in a hammock if required.
It is not a new idea: I too remember then as pied d'elephant- elephants foot. They were popular with alpinists for bivouacs
I wouldn't to be honest. If I had been in the rain all day, working hard and sweating as you would expect, the last thing I would want is to have to sleep in the jacket I may have been wearing. For the sake of a few grams I would stick with separate bag, bag liner and coat.
Now something I would be interested in would be a sleeping bag that is specifically designed for use in a hammock to defeat the having to carry a underquilt and/or using a sleep/inflatable/reflective heat type mats.
Sage advice a always Rik ;-)
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