I am always looking at existing baggage and pondering how I could use them for my bikes...

With a pair of the right backpacks you could sling them together and fashion a simple lower retainer to keep them secure... I have gone as far as thinking that that you could remove the straps if the bags were only going to be used for bike use.

You could also attach a basket to the top of the rear rack to hold a backpack or pick up some Wald collapsible baskets... these are just awesome.

I use motorcycle bags on my long bike and these are a sling type and have converted an old briefcase into a use-able bike bag by adding pannier clips... it holds an amazing amount of stuff and is sufficient for most of my commuting and small errands. The clips cost 6.00 and the install only took a few minutes.