"I wear insulated outerwear "
I normally wear an insulating layer (fleece, long underwear, premaloft, etc depending on the conditions). I find it cuts down on the bulk I need to pack (e.g. a lighter bag) and makes it a little easer to get out of my shelter (tent/bivi/hammock) when necessary.
Many years ago I saw a parka/sleeping bag combo that I thought was a good idea (the info said it was designed for the military of some European country). You could unzip the bag at the beltline and flip the bag up so that it attaches to some Velcro on your back (or remove the bag portion entirely). It was even waterproof so you did not need a shelter of any kind. It looked like a neat idea, but weighed a ton (I think it was cotton, wool and nylon).
One issue I see with the sleep suit, like mittens vs. gloves. Having less surface area makes mittens inherently warmer than gloves
dexterity is another matter. The same would apply here, the larger surface area would suggest that you could get equivalent warmth with a lighter conventional bag, but if getting up and moving around, or as you suggest not having to get into / out of a bag in a hammock
it could be a good idea. However I would side with Rich and suggest you could go with insulated outerwear and get similar warmth and even more versatility (probably at a lower cost).
However with a hammock, you still have the issue of the insulation between your body and the hammock, being compressed by your weight