There's a pattern for a poncho that accommodates a pack (similar to the Packa) at Backpacking Light but it's in the members section.
I carry a DriDucks poncho - twelve bucks, and it functions as an undercover for my hammock as well, or a place to sit, or rain gear, or a cover for my quilt.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzukihttp://hikeandbackpack.com