I'm rousing this thread from a five-year coma because I recently came up with what seems to me like a fairly neat solution to the Miyata 1000 seatpost binder-bolt problem, since the fitting on the seat lug is such that standard binder bolts don't fit.
The pictures pretty much tell the story. A standard M6 stainless-steel socket-head bolt from the hardware store just fits the lug. A 20 mm bolt is a couple of mm too long, so I used a 16 instead. It would have gotten a little more thread engagement by filing or grinding the 20 mm bolt down to about 18 mm, but the 16 seems fine. The original cylindrical Miyata nut was slotted for a screwdriver blade. That never worked very well (see previous posts in this thread), so in its place I used a mounting nut pilfered from a set of Jaguar brake pads. It's a perfect slip fit in the recess on the seat lug.
The result is a functional improvement on the original, since now you can use an allen wrench on both the nut and bolt. It also looks good, at least to me to me--more like something that could have come from the factory than the kludgy appearance you get from using a longer bolt, washers, and a conventional nut.
Also included is a gratuitous shot of the bike itself, which I just finished building up but have not yet ridden. You can also see the woodpile to my sauna, part of the sauna itself, and--in the first picture--some of my laundry hanging on the clothesline.