Hazy recollection of down pants that could join the legs as a quasi-bag using zippers or somesuch. Combined with a thick parka and maybe a simple bivy sack you have a kind of sleep system--more for climbing than leisurely camping, though.

"Elephant's foot" half-length sleeping bags that pair with a parka have always been around, although their popularity ebbs and flows. For anybody who owns a fat parka, that's a good option for 3-season sleep system with one-and-a-half, if not dual-use.

Then there are the bags and quilts that can be repurposed as down camp clothing. Jacks r Better and Feathered Friends come to mind.

Cheers,