on my pannier, there's an elastic strap with a ring on the bottom, and you catch the ring on a hook that's held down by the bolt that attaches the rack to the bike's eyelets. maybe you could use a similar hook on your bike, and exchange the hook on the pannier's elastic straps for a metal ring. it's really easy to attach the ring; you don't even have to touch it! you hold the top of the bag and get it to hook, then pull up and hook the top of the bag to the rack. it's beautiful in its simplicity.
(it's a banjo brothers waterproof pannier by the way. i've had it for a few months. it stays on pretty well, but on truly terrible roads it might not be as reliable. it does have some quality issues, such as fraying nylon in one or two spots, but it does what i ask it to. i hope to get the carradice bike bureau pretty soon, because i've heard such good things about it and it looks nice.)