I'm not worried about scorpions (or at least I wasn't -- thanks a lot PaHiker!) but I do worry about ticks. I was cowboy camping with just a plastic sheet in January in an area without established sites and very few level spots. All the spots I found late at night when I finally stopped looked as though the pine needle and leaf duff had been recently used as a bedding for large animals. I've never heard about problems with snakes approaching stationary people around here but deer ticks are a real concern.
My original thought for the foot was a rolled edge as you say with Velcro tabs or buttons but I may just seal the bottom up for simplicity and give up the "Gatewood Tunic" idea. If I just leave the bottom open it'll probably ride up in the night and I'll end up with my legs sticking out.
I've been waiting for a good rain since I put this thing together but the only rain we've had when I've been at home and available for this silliness has been a light drizzle. That afternoon I lay out in it for a half hour or so reading and it was fine. I set up an Outdoor Research bivy that a friend lent me and I tried that as well. I liked the extra space of mine and the ability to reconfigure the head material as an awning so that I could look out at the scenery or maybe even heat up a cup of coffee. With the OR bag when it's raining and there's no other protection overhead the only option is to zip it up and stare at blue nylon a few inches away. We'll see what happens in moderate wind and rain. Even if I stay bone dry inside it seems as though feeling the drops might get oppressive after a while.
I hadn't considered the effects of rolling around in the night. It's possible that I could bring the whole awning sheet in the head area down. Maybe when it's sleep time I just pull it over my head and curl up burrito style. It sounds as though some backyard testing it in order, but I'll probably just try it on an overnighter in a few weeks when it probably won't rain and see what happens. The plan I have in mind has me sleeping about a mile from a trail shelter so failure won't be too expensive.
The head area is still a work in progress. The material started out as a 9x5 foot piece of Tyvek and a 2x2 sheet of Polycro (or Polycryo ... whatever). I rolled and glued the bottom 5 feet and cut sideways at that point in either direction so that when layed out it looks like a "T" with the vertical bar 2 1/2 feet wide double-thickness and the horizontal bar 5 feet wide by 4 feet high single thickness. Then I glued a 2-foot by 2-foot piece of polycro to the bottom centered on the seam, where I'd cut the Tyvek horizontally, to extend the ground sheet from five to seven feet. This means that the contraption is not roll-agnostic. I lie down and drape floppy 5-foot by 4-foot head section over a few hooped branches that I stuck in the ground. As with a tarp, I can tug and fold into many configurations. I don't plan to stake it, just weigh bits down with my pack (used as a pillow) and maybe some rocks or something.