Great bunch of responses, much to ponder, thanks.
David beat me to the polycryo poncho I see. Nice work though. Looks like you put snaps on the sides? What kind, did they work? You can pull them apart without tearing the poly? You pretty happy with the poncho's functionality, other than the hot hood? Did you shower-test it? How does it function as a groundcloth, with the head hole? I thought a groundcloth was compromised if it had holes in it.
Nick: I do own a couple of Tilly hats, which I use for outdoor work, I never considered them for backpacking due to the cotton and 5.5 oz weight. Maybe I already own the solution, but I'm guessing your LT6 is a nylon hat? Yes, cactus will certainly shred polycryo. I'm not an accomplished and intrepid hiker like you, so don't plan much use for the poncho at all; for me this is an emergency contingency, pack-ballast if you will.** If I thought I'd be in the rain frequently, I'd go with a decent jacket. (currently I own frogg toggs which I'd take if rain were in the forecast.) But at some point you have to trust empiricism, and I just don't see much rain where I hike, so have targeted this area for lightening. I think we're all agreed that a poncho is not an ideal piece for hiking (altho it does ventilate) regardless of the material. So: This polycryo hat cover idea of yours, a good one. Do you envision elastic around the brim? How would you batten it down?
Adam: Yes, a windshirt (a Houdini) will be worn underneath; always carried and will be first on if rain starts to fall. The poncho is for over that. ((PS: Sent you a PM regards our previous conversation.))
Patrick: I will definitely try the showercap idea. That's the fastest solution!
Ken: Yes, my intelligence deficit, again. I'm not quite as bright as my avatar photo makes me look. We do what we can to cope.
Bob: If you don't stop controlling my thoughts, I will have to use the aluminum foil in my hat. Please stop, so I can hike lighter.
** Honestly, I'm extremely tempted to just use my tent's fly as my contingency poncho. Still need a rainproof hat, though.