I have that bag on my single. We have a similar nonexpandable version on our tandem. It's thinner and it doesn't extend down as far below the frame as the expandable one does. Even the thinner one hit the rear wheel on our tandem so I just bent the bag frame up enough so that the bad didn't hit (I put it in a vise and pushed - it's aluminum which doesn't like to be bent cold very much but I didn't move it very much). Both bags are the older style that fasten with velcro that another poster warned you about. The thinner model has never loosened up in 4000 miles on the tandem this year but I rode over 10,000 miles two summers ago including a cross country trip on my single and the bigger - heavier bag did loosen up - every day I would check it multiple times. My wife had the thinner one on her single for the same trip and it never loosened up. As another poster mentioned, they have now improved / changed the attachment.
http://www.detours.us/ is the company that makes and / or markets the bags in the U.S.
The company that makes the KlickFix attachment mechanism for these bags is Rixen Kaul from Germany. They make a wider variety of bags and may very well make the bag for detours to their specifications but I like the Rixen Kaul designs better - at least from what I can tell from looking at images on the computer. If you go to the Rixen Kaul site and look at New products you will see a bag that is suppose to come out in 2007 that doesn't extend much below the frame - it is for low clearance situations. Here's the English language versionlink:
http://www.klickfix.com/neue.htm
They have all sorts of bags that use their quick release mechanism. They make a Bento box style bag with a quick release for example - good stuff in my opinion. To order an actual Rixen Kaul bag instead of a detours one, I think you may have to go to an online United Kingdom shop - I haven't seen them at any U.S sites but maybe I just haven't looked hard enough.
My wife loves her Tamer Pivot Plus stoker seat post. She has a tender lower back with a sciatic nerve which gets pinched - no extra shocks if we can help it. On our bumpy roads (bumpy from lumpy repairs to winter freeze and thaw damage) she would not be able to avoid problems without the post because I wouldn't be able to see and call out in time all the bumps we hit. Her single Klein she rode cross country has a little shock absorber mechanism in the seat stay up near the seat post. It's used by Trek now on some of their bikes - I believe they also used it on the bikes for the Paris-Roubaix race. My wife definitely wants her shock absorption seat post. The Tamer Pivot plus can be fine tuned in addition to the mechanism not taking up as much room as the regular thudbuster mechanism but I believe that thudbuster also makes a model with a smaller mechanism intended for road bikes.
You could buy a very cheap non-shock-absorbent stoker seat post ($20) to let her try out.
Good luck.