If it's a trip where I expect just lots and lots of rain, I probably wouldn't go this route, but for one where it's lighter, on/off rain or just "possible" rain I'd consider a gatewood cape as a great choice for such a day hike. Perhaps add some tyvek sleeves if you're so inclined. The G.C. is not, IMO, optimal rainwear, but it's okay, and it pitches pretty fast as a floorless tent. My wife and I can cram in there for a dayhike break of the type you describe.
Of course, if it's raining much when I put it up or take it down I'll likely get a bit wet in so-doing, but no big deal IMO. If that really concerned me I could also bring along a very light disposable poncho.
Alternatively, a more conventional poncho tarp, such as a golite poncho could work in similar fashion.
And a very light rain jacket that can double as essentially a windshirt could be a worth while thing to augment this. Or just a straight windshirt.