Well, first, don't mention your idea at Bikeforums unless you've got the time to spare.
http://www.bikeforums.net/showthread...02#post1981102
There's always the Wilderness Safepacker. I keep saying I'm going to get one of these for fanny pack carry in Illinois, but I haven't taken the plunge yet. $50 is pricey for me, but it would give a lot of options as far as how to attach or carry.
http://thewilderness.com/catalog/pro...products_id/18