But for what reason please?
Doesn't this thing has a zipper that goes around the foot section too?
If you 'wear' it as a poncho-thing, you would still need some insulation around your legs for a comfortable night - at least in cooler, temperate climes.
It seems that the criticism aimed at the Swagman revolves around it being too tight around the shoulders when zipped up and used in 'sleeping bag mode'. Even the compact American gentleman in the review found it a little too snug for comfort, when zipped up. Admittedly, you could use it partially unzipped but then you would lose out on the insulation qualities of it as a 'bag' - and, as I'm sure many of us know, being cold at night prevents one from sleeping well. By making use of a half-bag/'pied d'elephant' along with the Swagman in 'poncho mode', you would keep warm while still having a piece of gear that can be used as blanket and underquilt in other scenarios.
I suppose therein lies its shortcoming as jack of all trades, master of none. For reasons that I cannot fathom, I'm still interested in one... Weird!