gregphoto,greetings from another photographer and once neighbor of yours (chief photographer, Wilmington, DE News-Journal in 1960s). I'm afraid I'm late to the party (long hike followed by long hospital stay -- mercifully unrelated) but I think I can offer you some encouragement with your plan. Big difference, cameras were my work, rarely took them on the trail.

First, I do not know the specific equipment you are considering,but I do know bivys. and have the experience to offer "proof of concept."

I have backpacked since c.1950, and have essentially always used some version of the bivy/poncho system. Rarely ever carried a tent, though I've used several types of bug-net.

I started with the military blanket bag and Egyptian cotton (water resistant) cover. Military poncho rigged lean-to usually was enough to stay dry enough and shelter pack and boots (if the wind direction held). Later, I used a Thos. Black & Sons sleeping bag cover ("Icelandic"-water resistant) as a bivy with a very light down bag. Used lightweight commercial poncho just as above.

In 1996 I thru-hiked the AT for the first time. I used an Integral Designs South Col Bivy. It was 27 oz., breathable top, waterproof bottom. It had bug-net face, and could be completely closed (I never did). I was delighted with it, and rarely felt the need to rig my poncho. However, one night, the poncho was committed to another use, and I was out, wide open through a very long night of absolutely driving rain. I was never able to identify for certain where the rain got in, but I slept in a puddle. Fortunately, it was not very cold, and I was using a synthetic bag at the time. I found I had to open the side zip a bit to breath. My pack and boots were deeply soaked.

Like you, I used layers and a very light bag (Cascade Designs, 40 degrees fahrenheit, bit over a pound). I never got cold, so I cannot be exact, but it was clear that the bivy was giving me several extra degrees of warmth, probably about six or maybe even eight. I was warm when I reached out and grabbed a frozen water bottle, twice.

In 2006 I thru-hiked again. By now I had hiking poles, a very fine, well-thought-out oversize poncho and very light bivy (also very well thought-out, about 5 oz, breathable top, waterproof bottom) both by Ron Bell of Mountain Laurel Designs (also a former photographer). The poles and Ron's innovative poncho gave me several new ways to pitch quickly, so I have not tried the bivy in an extended downpour (it did hold up to several unexpected and fairly heavy showers). If the weather seems "iffy" I will stake down the bottom corners of the poncho and accordian-fold it over the foot of the bivy so I can pull it up for serious rain or to cut serious wind.

In reasonable weather, I still do as I have for years, just throw down my bedroll, maybe rock down the poncho over my boots and pack, and tuck in. Usually, I leave the bivy partly open. Incidentally, did the same thing as a journalist with Special Forces and Ranger unit training exercises. With the Marines, active duty, it was usually just roll up in the poncho and poncho liner, similar, but lots of condensate.

I think this constitutes enough experience to justify a few flat statements:

First, all bivy's (even cotton) will condense to some degree, in the right circumstances, but working the way I have described, I think I remember it being a serious nuisance about once(it's only a problem if it keeps you awake, or soaks your sleeping bag dangerously).

Second, I can't quite imagine that any bivy would be totally, completely waterproof in an extended downpour (you have to breath, get in and out, etc.), but the South Col is close. (I suspect that Ron Bell's little gem is is fairly close, but not quite ready to push it).

Third, I think you are on the right track for sure. The tripod with poncho arrangement gives you shielded breathing room in a storm, and solves the other weaknesses of the bivy alone, shelter for pack and boots, and, using poles and a couple of stakes would also allow you to easily solve another problem, shelter for your kitchen. I say go for it!!!

I also note that what I'm saying and suggesting is not very far from what wiser heads have said to you above. But I do like your idea.