I have actually thought about designing a frameless pack that would accomplish this. The idea came to me with my fanatic fringe thmpson peak pack; specifically the extension collar. I figured a long enough"collar" that would fold down into the the pack itself could be extended to make the pack a full length bivy. I have made some prelimnary drawings but never put scissors to cloth since I use a hammock and a bivy sack would not be useful to me.