Richard and Vick,
Thanks for the responses. I have the ultralight extension poncho, too, and might eventually attempt to construct one of my own.
We had heavy rain for the first time in months today. It was 52F and pouring, so I went for a walk to test my seam sealing. No leaks, but I definitely will leave my hands inside at these temperatures.
If you use the poncho as shelter, what's the setup procedure in heavy rain? My hypothesis is to start by stringing the ridgeline and having stakes ready, but hiker and gear going to be exposed for a few minutes when removing and tying out the poncho. Maybe cover them with Tyvek groundsheet until the poncho is rigged, but then the groundsheet is wet, unless you put it bottom side up for those few minutes?
Am I getting close?