Quick follow-up for those who were on the edge of their seats waiting to hear if my Xootr Swift/Planet Bike fender conundrum was all better (sidenote to those people: good grief, get a life): Yup. See above.
As I said I would (because I meant what I said and I said what I meant, an elephant's faithful one hundred percent), I drilled out the rivets holding the fender bracket in place, drilled new (and crooked -- the charm of handmade) holes down where the bracket should have actually been installed in the first place, used nuts/bolts to secure the bracket down there, and taped over the old holes. Result? Rock-solid fenderage. No flop, flutter, or flaccidity. If you look close, it has the unmistakable style of what my straw-hatted and black-bumpered ancestors called "Mennonite Maintenance", but considering all the stuff those Mennonites could fix with string, spit and duct tape, I'll take that as a compliment.
The resulting fenders work as well as any plastic fenders I've used. Their length around the tiny wheels provides absolutely outstanding coverage, keeping road slime off the drivetrain in the front and curing the soaked sphincter issue I was getting from my old clip-on rear fender. It's a good thing.