Okay, from my experience, a poncho and a fleece blanket, at least the ones I use really don't compare well insofaras what I would prefer for a sleeping bag replacement. IMHO, the right fleece blanket is a most suitable replacement, while the poncho is marginal at best.

I refer you to the waterproof fleece products made by riverswest. I've recommended these in the past, and I stand by the product still. These are not your momma's fleece products. They are heavy duty, they will keep you warm and dry, and they will take a helluva lot of abuse. Their product gaurantee cannot be beat. In addition to a number of outer garments I own and have worn in some of the worst conditions the Colorado Rockies could throw at me in February while hunting coyotes, as well as trout fishing in the Blue mountains in spring with torrential rains and 40 degree winds churning the air, I also recently acquired what they refer to as an artillery blanket. It is big enough for me to completely bundle up in ( I am a little bit bigger than you, Blast, both in height and girth), and it was a most excellent ground pad for a recent shooting exercise in miserably wet conditions.

The fleece is actually two thin but tightly woven layers that are bonded together with a breathable but waterproof barrier and reinforcing material. This makes them considerable denser than the typical fleece, but also adds to their durability. The material is nonetheless quite comfortable against the skin, and fairly quiet. It breaks in over time, becoming more supple and less rigid after a few uses and a couple washings.

Check out their website and see for yourself. My product endorsements are very rare, but when I find something that works above and beyond my expectations, something I know I can rely on, I am quite willing to let others know.

http://www.riverswest.com/