It seems to me that the only pack-shelter integration that makes sense is using the shelter frame elements and sleeping pad to provide structure to the backpack.

At first glance, it would seem that it might be worth exploring combining a pack integrating a half-bivy with a separate half-length, hooped tarp shelter for UL backpacking. You would get the best of both worlds : the convenience of a tarptent for unpacking, entering and exiting the shelter under precipitation, get (un)dressed, etc. and compact lightweight-ness of a bivy bag. In addition, on its own, the pack functions as an emergency bivy. The classic version of the mountaineering backpack Karrimor Alpiniste was just this combination of pack and half-bivy. In normal backpack use, the bivy top was folded down inside the pack. With the bivy part deployed, it covered you only to the armpits. Without a tarp, it worked as an emergency bivy and needed to be complemented by a mountaineering jacket (battened down as tight as possible). The removeable frame was made of two aluminum stays encased in a foam pad and could be used in combination with rope and slings as an emergency sleeping pad.

However, all you save using this approach compared to one where the bivy is not integrated into the pack is the packís surface area in a lightweight fabric equivalent and a 2 sq ft bit of foam pad; ie, next to nothing; and thatís assuming that you are not squandering that weight saving with extra zippers, buckles, etc. In addition, you lose the flexibility of a separate bivy bag and have to worry about the integrity of your shelter getting compromised by rocks and branches poking at the pack. Karrimor still has the Alpiniste in its line-up but it no longer incorporates a half-bivy and hasnít for many years. I guess that even in the UL world of mountaineering, youíre still better off having to choose between taking a full UL bivy and taking no shelter at all (other than regular mountain pack, boots and clothes) rather than having a half-bivy permanently attached to your pack.