I really do
see the utility of ponchos. They don't work well in the wind, and they're not particularly comfortable to wear when hiking, especially in rough terrain. But as you mention, they have multi-use capability. Plus, they're cheap. Ponchos typically are made of coated nylon, an extremely inexpensive material. You could buy a "surplus" military poncho for as little as $10.
But, if anybody makes a Gore-Tex parka, that's news to me. There's really no reason to use an expensive material in a poncho. For one thing, ponchos are usually so big that the cost of the fabric would be high. For another, the poncho doesn't really enclose the wearer; they're designed to just drape over the body. So (again, as you note) they afford plenty of air circulation. The spiffiest poncho I can find is one made by Mountain Equipment Co-op in Canada. Called the Torrent Rain Poncho, it's made of coated nylon twill and sells for about $55 U.S. (www.mec.ca).
I should think a Gore-Tex poncho would be something any amateur sewer worth their Singer could pull off. All you'd need is a big sheet of the stuff (in three-ply laminate form). This can be purchased as sheetgoods at Seattle Fabrics; you can order direct from them, or maybe they can point you in the direction of someone closer to home. In its simplest form, just cut a rectangle, sew the edges, then cut a head-hole in the middle (I'll assume you'll use a rain hat, too). Making one with a hood would be the next reasonable step. Now there's a project for you!
Lead Photo: courtesy, MEC