I'm glad my experiment was somewhat useful. When my neighbors ask, I'll just blame the whole thing on BPL. Or perhaps take the Peter Venkman route from Ghostbusters: "Back off, man. I'm a scientist."
*Dimensions on the bags are listed as 2'6" x 2'9", 76 cm x 83 cm, 30 Gal., 113 L, 1.3 Mil.
*Padding was a regular length Thermarest Ridgerest Deluxe, a short inflatable from REI, and some 3/8 closed cell foam from the hardware store. Seemed adequate.
*I second the opinions on sleeping cool, and have found (as others have noted) that if there is any overheating, the VB lets you know pretty quickly.
*Pillow was not in the VB, but inside the SB. The VB drawcord was roughly around my neck, made more comfortable by the fleece neckwarmer. If I was to cinch the VB more around my face, and enclose my head, I would have to reposition the drawcord relative to its current position. I would have to give some thought to the advantages and disadvantage of this. My sense is that the head alone probably gives off relatively little moisture, and what moisture it does give off is mostly from breathing, which obviously can't be in the VB! Also, even with a nicely tightened SB hood around the face, there is probably still some opportunity for moisture to leak out that opening, unlike other areas of the SB. So at this point, even simply for psychological reasons, I would prefer to keep the liner around my neck, retain some connection to my surroundings through all that fabric, and leave the head outside the liner: then throw on a warm hat, balaclava, etc. In a tent, w/o a bivy, I can picture a simple, small piece of material with a face hole and perhaps some elastic to hold it in place, but imagine that it might not be worthwhile in many situations.
*Respiration moisture did condense around my face on the SB, though not enough to significantly influence loft or wet the down. I remember Andrew Skurka mentioning this issue, though I don't recall he discussed in detail how he resolved it other than by drying. With the bivy, the trick is figuring out a way to position the bivy (or any other piece of material) reliably in place close to the "airhole" before sealing oneself in, at which point you are encased in the whole rig. For many trips, with a decent DWR on the bag, this would probably not become a serious issue- and any drying opportunities could be focused on that one small area of the SB. Other BPL members have suggested that breathing through fabric like a neck warmer or balaclava captures at least some of the moisture, and makes for warmer, more humid air to breathe. I did breathe through fleece and didn't mind it, but this solution does not solve the problem entirely. As Roger mentioned, sometimes you are going to be snowing on yourself regardless!
*I reinforced with duct tape in a few spots- the end seam of the liner down by my feet, as well as the edges near where my elbows would usually fall- I sleep with wide elbows. Perhaps I will post a picture of the next version- this one was slapped together and its lack of aesthetic merit might lead others to doubt the scientific value of my claims :). The path for the drawcord was made simply by folding the bag back on itself by 2 inches or so to create a channel that was then taped shut. I found it very helpful to place the drawcord inside the liner. I did keep some excess material above the drawcord- once you were certain about the length you preferred, you could simply make the drawcord the top of the liner, therefore making the issue of inside or outside irrelevant.
*It would stand to reason that stresses on a VB liner would vary depending on sleep style, size of person, etc. Once the basic design was tested with garbage bags, I'm sure a lighter, much more durable version could be constructed by someone with much greater knowledge of materials and a hand for sewing.
*There are other options that probably start to sound impractical/ludicrous but might be fun to at least consider, such as elasticized arm holes in the side of the bivy allowing some use of hands while also sealing the upper torso to the neck. VB "overalls" with integrated feet, leg, torso, and arm coverage (this approaching closer to VB clothing, but a bit more integrated), like kids pajamas. Perhaps even some way to take advantage of body heat while making a VB stew? Bon appetit?