This has got to be a common problem, but I've never encountered it before. Apparently the previous owner of the Univega I built up this winter torqued mightily on the wrong side of the seatpost binder bolt in trying to loosen it, and sheared off the slot in the seat lug that engages the tab in the binder bolt. Years of subsequent overtightening allowed the head of the bolt to burrow several mm into the "ear" on the lug. (Both ears themselves are intact--just sort of reamed.)
The obvious way to deal with this would be to slip a washer over the bolt on that side (maybe have to slot it to engage the tab on the bolt half), and use two allen wrenches to loosen and tighten the bolt from here on. I imagine that will work and it's probably what I'll do. But just wondering if someone else out there has a more elegant solution.
JV